[Fix] (DirectGLES): apply the read buffer when one FBO is bound as both draw and read

- SyncCurrentFBO skips the READ-target pass when the same GL FBO is bound as
  both draw and read (the common GL_FRAMEBUFFER case), but the read buffer
  (glReadBuffer) is only applied inside SyncToBackend's READ path — so the skip
  silently dropped every glReadBuffer change, leaving the backend read buffer
  stuck at COLOR_ATTACHMENT0.
- Extract the read-buffer application into BackendFramebufferObject::
  SyncReadBufferToBackend and invoke it from the skip branch (target == Read)
  as well as from SyncToBackend, so reads always target the right attachment.
- Bind the backend FBO as READ inside the helper before glReadBuffer, since the
  skip path only bound it as DRAW.
- Fixes KHR-GL3x.draw_buffers.draw_buffers_1 (reading COLOR_ATTACHMENT1 while
  the FBO stays GL_FRAMEBUFFER-bound returned attachment 0's value); the render
  was already correct, only the readback resolved the wrong attachment.

MobileGL/MG_Backend/DirectGLES/DirectGLES.cpp

@@ -531,6 +531,18 @@ namespace MobileGL::MG_Backend::DirectGLES {
 
                 if (currentFBO.get() == lastUpdatedFBO) {
                     MGLOG_D("Draw FBO and read FBO are the same, skipping sync.");
+                    // The attachment/draw-buffer work was already done for this GL FBO as the DRAW
+                    // target, but the read buffer (glReadBuffer) is READ-target-specific and would
+                    // be dropped by this skip. Apply it so reads target the right attachment (e.g.
+                    // KHR-GL33.draw_buffers reads each COLOR_ATTACHMENT while the FBO stays bound as
+                    // GL_FRAMEBUFFER — without this every glReadBuffer is a no-op and all reads hit
+                    // COLOR_ATTACHMENT0).
+                    if (target == FramebufferTarget::Read) {
+                        const auto& syncedFBOIt = g_backendFramebufferObjects.find(currentFBO.get());
+                        if (syncedFBOIt != g_backendFramebufferObjects.end() && syncedFBOIt->second) {
+                            syncedFBOIt->second->SyncReadBufferToBackend(currentFBO);
+                        }
+                    }
                     g_fboSyncedObjectVersions[SizeT(target)] = objectVersion;
                     g_fboSyncedObjects[SizeT(target)] = currentPtr;
                     continue;

MobileGL/MG_Backend/DirectGLES/Managers.cpp

@@ -2535,6 +2535,28 @@ namespace MobileGL::MG_Backend::DirectGLES {
             return false;
         }
 
+        void BackendFramebufferObject::SyncReadBufferToBackend(
+            const SharedPtr<MG_State::GLState::FramebufferObject>& stateFBOObject) {
+            if (!stateFBOObject) {
+                return;
+            }
+            auto frontendReadBuf = stateFBOObject->GetReadBuffer();
+            if (frontendReadBuf == m_frontendReadBuffer) {
+                return;
+            }
+            m_frontendReadBuffer = frontendReadBuf;
+
+            GLenum glBackendReadBuffer = GetBackendAttachmentType(frontendReadBuf);
+            if (m_backendReadBuffer != glBackendReadBuffer) {
+                m_backendReadBuffer = glBackendReadBuffer;
+                // glReadBuffer targets whatever FBO is bound to GL_READ_FRAMEBUFFER. When this is
+                // reached from SyncCurrentFBO's "same FBO as draw" skip path the backend FBO was
+                // only bound as DRAW, so bind it as READ first to route the read buffer correctly.
+                Bind(FramebufferTarget::Read);
+                g_GLESFuncs.glReadBuffer(glBackendReadBuffer);
+            }
+        }
+
         void BackendFramebufferObject::SyncToBackend(
             const SharedPtr<MG_State::GLState::FramebufferObject>& stateFBOObject, FramebufferTarget asTarget) {
 #ifdef TRACY_ENABLE
@@ -2616,16 +2638,8 @@ namespace MobileGL::MG_Backend::DirectGLES {
 
             // 2. Remap read buffer. glReadBuffer writes the READ-bound FBO's state, so
             // only apply (and stamp the memo) when this object is bound as READ.
-            auto frontendReadBuf = stateFBOObject->GetReadBuffer();
-            if (frontendReadBuf != m_frontendReadBuffer && asTarget == FramebufferTarget::Read) {
-                m_frontendReadBuffer = frontendReadBuf;
-
-                GLenum glBackendReadBuffer = GetBackendAttachmentType(frontendReadBuf);
-
-                if (m_backendReadBuffer != glBackendReadBuffer) {
-                    m_backendReadBuffer = glBackendReadBuffer;
-                    g_GLESFuncs.glReadBuffer(glBackendReadBuffer);
-                }
+            if (asTarget == FramebufferTarget::Read) {
+                SyncReadBufferToBackend(stateFBOObject);
             }
 
             // -------------------- Attach texture to backend FBO -----------------------

MobileGL/MG_Backend/DirectGLES/Managers.h

@@ -375,6 +375,10 @@ namespace MobileGL::MG_Backend::DirectGLES {
             BackendFramebufferObject();
             void SyncToBackend(const SharedPtr<MG_State::GLState::FramebufferObject>& stateFBOObject,
                                FramebufferTarget asTarget);
+            // Apply only this FBO's read buffer (glReadBuffer) to the backend. Split out so it can
+            // still run when SyncCurrentFBO skips the READ-target sync because the same GL FBO is
+            // bound as both draw and read (otherwise glReadBuffer changes would be silently dropped).
+            void SyncReadBufferToBackend(const SharedPtr<MG_State::GLState::FramebufferObject>& stateFBOObject);
             void InvalidateSyncedState();
             Uint GetBackendFramebufferId() const { return m_backendFBOId; }
             void Bind(FramebufferTarget target) const;