[Feat] (Loader): detect GL_OES_viewport_array in the GLES capability scan

MobileGL/MG_Util/BackendLoaders/OpenGL/Loader.cpp

@@ -932,6 +932,9 @@ namespace MobileGL::MG_Util::BackendLoader {
                 if (std::strcmp(extension, "GL_EXT_clip_cull_distance") == 0) {
                     caps.SupportsClipDistance = true;
                 }
+                if (std::strcmp(extension, "GL_OES_viewport_array") == 0) {
+                    caps.SupportsViewportArray = true;
+                }
             }
         }
         // The pointer check on top of the extension check makes each flag sufficient on its own
@@ -989,6 +992,8 @@ namespace MobileGL::MG_Util::BackendLoader {
         MGLOG_I("    base instance (EXT_base_instance; emulated by attribute offsets when absent): %s",
                 caps.SupportsBaseInstance ? "yes" : "no");
         MGLOG_I("    clip distances (EXT_clip_cull_distance): %s", caps.SupportsClipDistance ? "yes" : "no");
+        MGLOG_I("    viewport array (OES_viewport_array; gl_ViewportIndex collapses to viewport 0 when absent): %s",
+                caps.SupportsViewportArray ? "yes" : "no");
 
         // LOAD-BEARING STRING, not just a banner. android-plugin/trace-replay-ci.sh's
         // is_angle_surface_lost() greps mobilegl.log for exactly "OpenGL ES capabilities:" to

MobileGL/MG_Util/BackendLoaders/OpenGL/Loader.h

@@ -1182,6 +1182,21 @@ namespace MobileGL {
             // compile and the per-distance enables have nowhere to go - clipping silently never
             // happens, which is exactly what KHR-GLxx.clip_distance.functional catches.
             Bool SupportsClipDistance = false;
+            // GL_OES_viewport_array is present: the driver knows gl_ViewportIndex in ESSL - and
+            // only then. ESSL has no core spelling for it at ANY version, while SPIRV-Cross prints
+            // the identifier bare and requests nothing for it (contrast gl_Layer, which it backs
+            // with GL_NV_viewport_array2 on ES), so the `#extension GL_OES_viewport_array :
+            // require` line has to be inserted into the emitted source - see
+            // RequestViewportArrayExtension. Without the extension the stage does not compile at
+            // all and the whole program becomes unusable, which on DirectGLES means every draw
+            // using it silently renders nothing; LowerViewportIndexPass is the fallback that
+            // demotes the builtin so the program still links and degrades to viewport 0.
+            //
+            // Extension string only, deliberately: DirectGLES does not call any of the indexed
+            // OES entry points yet, so there is no pointer to require. When that forwarding lands
+            // this must gain the pointer check as well - the rule everywhere else in this struct,
+            // because eglGetProcAddress can hand back a stub that silently drops every call.
+            Bool SupportsViewportArray = false;
             // GL_RENDERER contains "ANGLE".
             Bool IsAngleRenderer = false;
             // GL_RENDERER contains both "ANGLE" and "llvmpipe".