[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.cpp

@@ -133,6 +133,16 @@ namespace MobileGL {
                 return m_parameters.PolygonModeBack;
             }
 
+            void RenderState::SetPrimitiveRestartIndex(Uint32 index) {
+                if (m_parameters.PrimitiveRestartIndex == index) return;
+                m_parameters.PrimitiveRestartIndex = index;
+                ++m_version;
+            }
+
+            Uint32 RenderState::GetPrimitiveRestartIndex() const {
+                return m_parameters.PrimitiveRestartIndex;
+            }
+
             void RenderState::SetPointSize(Float size) {
                 if (m_parameters.PointSize == size) return;