[Fix] (MG_State): detach a deleted texture from the framebuffer that is bound

GL 3.3 core 4.4.2: deleting a texture whose image is attached to the framebuffer
currently bound acts as if FramebufferTexture* had been called with texture zero for
every attachment point it occupied there. Framebuffers that are not bound keep the
orphaned attachment, so only the bound ones are touched.

MobileGL unbound a deleted texture from every texture unit and image binding but
left framebuffer attachments alone, so the framebuffer went on holding the dead
texture alive as its attachment and reads through it returned that texture's
contents rather than those of whatever the application put in its place - and since
the deleted name usually comes straight back out of the next glGenTextures, the two
are indistinguishable from the outside.

MobileGL/MG_State/GLState/Core.cpp

@@ -241,6 +241,32 @@ namespace MobileGL::MG_State {
         }
 
         void GLContext::MarkTextureObjectForDeletion(Uint index) {
+            // GL 3.3 core 4.4.2: deleting a texture whose image is attached to the framebuffer
+            // that is currently bound acts as if FramebufferTexture* had been called with texture
+            // zero for every attachment point it occupied there. Framebuffers that are NOT bound
+            // keep the orphaned attachment, so only the bound ones are touched.
+            //
+            // Without this the framebuffer object goes on holding the deleted texture alive as its
+            // attachment, and a later read through that framebuffer returns the dead texture's
+            // contents rather than those of whatever the application put in its place - the name
+            // it deleted usually comes straight back from the next glGenTextures, so the two are
+            // indistinguishable from the outside (KHR-GL32.packed_pixels read a stale gradient).
+            if (const auto& textureObject = m_textureState.GetTextureObject(index)) {
+                for (SizeT targetIndex = 0; targetIndex < SizeT(FramebufferTarget::FramebufferTargetCount);
+                     ++targetIndex) {
+                    const auto& framebuffer =
+                        GetFramebufferBindingSlot(static_cast<FramebufferTarget>(targetIndex)).GetBoundObject();
+                    if (!framebuffer || framebuffer->IsDefaultFramebuffer()) {
+                        continue;
+                    }
+                    const auto& attachments = framebuffer->GetAllAttachmentObjects();
+                    for (SizeT i = 0; i < attachments.size(); ++i) {
+                        if (attachments[i].IsTexture() && attachments[i].GetTexture() == textureObject) {
+                            framebuffer->Detach(static_cast<FramebufferAttachmentType>(i));
+                        }
+                    }
+                }
+            }
             m_textureState.MarkTextureObjectForDeletion(index, IsRelaxedSemanticsActive());
         }