[Feat] (MG_Impl/GLImpl, MG_State): implement glPrimitiveRestartIndex

Store the primitive restart index as render state and report it through
glGetIntegerv(GL_PRIMITIVE_RESTART_INDEX), replacing the stub and the
hardcoded 0 in the getter.

- RenderState gains a PrimitiveRestartIndex field (default 0) with
  set/get accessors and GLContext wrappers.
- glPrimitiveRestartIndex accepts any GLuint and generates no error.
- glGetIntegerv(GL_PRIMITIVE_RESTART_INDEX) now reads the stored value.

This is the state layer only. The backends do not yet honor an arbitrary
restart index at draw time -- Vulkan and GLES support only the fixed
all-ones restart value (GL_PRIMITIVE_RESTART_FIXED_INDEX) -- so a non-
default index is tracked and queryable but not yet applied to indexed
draws.

Tests: RenderStateSanity round-trip (default 0, mid value, and the full
32-bit range). Full SanityTest sweep green (31/31).

MobileGL/MG_State/GLState/RenderState/RenderState.h

@@ -269,6 +269,10 @@ namespace MobileGL {
         GLenum PolygonModeFront = GL_FILL;
         GLenum PolygonModeBack = GL_FILL;
 
+        // Primitive restart index (glPrimitiveRestartIndex); consumed when GL_PRIMITIVE_RESTART is
+        // enabled during an indexed draw. Default 0.
+        Uint32 PrimitiveRestartIndex = 0;
+
         // Scissor
         Bool ColorLogicOpEnabled = false;
         Bool DebugOutputEnabled = false;
@@ -326,6 +330,8 @@ namespace MobileGL {
                 void SetPolygonMode(GLenum front, GLenum back);
                 GLenum GetPolygonModeFront() const;
                 GLenum GetPolygonModeBack() const;
+                void SetPrimitiveRestartIndex(Uint32 index);
+                Uint32 GetPrimitiveRestartIndex() const;
 
                 // Capabilities
                 void SetCapability(CapabilityInput cap, Bool enabled);