[Fix] (Backend): resolve primitive restart per draw - never for a non-indexed one, and never on an index the type cannot hold

MobileGL/MG_Util/Metrics/BufferMetrics.cpp

@@ -10,6 +10,15 @@
 
 namespace MobileGL {
     namespace MG_Util {
+        Uint32 FixedRestartIndexForGLType(GLenum indexType) {
+            switch (indexType) {
+            case GL_UNSIGNED_BYTE: return 0xFFu;
+            case GL_UNSIGNED_SHORT: return 0xFFFFu;
+            case GL_UNSIGNED_INT: return 0xFFFFFFFFu;
+            default: return 0;
+            }
+        }
+
         SizeT GetGLTypeSize(GLenum type) {
             switch (type) {
             // Scalars

MobileGL/MG_Util/Metrics/BufferMetrics.h

@@ -12,5 +12,19 @@
 namespace MobileGL {
     namespace MG_Util {
         SizeT GetGLTypeSize(GLenum type);
+
+        // The largest value an index of this type can hold, which is also the value
+        // GL_PRIMITIVE_RESTART_FIXED_INDEX (and its GLES/Vulkan equivalents) restart on. Zero for a
+        // type that cannot index at all.
+        //
+        // Shared rather than re-derived per backend on purpose: three places have to agree about
+        // what an index of this type can be - whether a rewrite is needed at all, what the rewrite
+        // compares against, and whether the driver should be told to restart. GL 4.6 core 10.3.6
+        // compares the FETCHED index, zero-extended, against the full 32-bit
+        // PRIMITIVE_RESTART_INDEX, so a restart index greater than this value matches no index and
+        // the draw restarts nowhere. Truncating it to the type's width instead - which one of these
+        // three places used to do - turns a legal vertex index into a restart.
+        Uint32 FixedRestartIndexForGLType(GLenum indexType);
+
     } // namespace MG_Util
 } // namespace MobileGL