[Feat] (MG_Impl/GLImpl, MG_State): implement glClampColor and glPolygonMode

Fill the two empty // TODO state handlers with GL 3.3 Core-conformant
behavior, backed by new RenderState fields and glGet* read-back.

glClampColor:
- Accept only GL_CLAMP_READ_COLOR (compat GL_CLAMP_VERTEX/FRAGMENT_COLOR
  rejected); clamp is one of GL_TRUE / GL_FALSE / GL_FIXED_ONLY. Note the
  Khronos man page wrongly omits GL_FIXED_ONLY from the accepted set, but
  it is legal AND the default, so it is accepted here.
- Default GL_FIXED_ONLY; both error paths are GL_INVALID_ENUM with no
  state change. glGetIntegerv returns the raw tri-state enum; GetFloatv/
  GetDoublev widen it and GetBooleanv converts nonzero to GL_TRUE via the
  existing fall-through, so one GetIntegerv case serves every getter.

glPolygonMode:
- Core accepts only face == GL_FRONT_AND_BACK (GL_FRONT/GL_BACK were
  removed in 3.1 core); mode is GL_POINT / GL_LINE / GL_FILL. Both errors
  are GL_INVALID_ENUM with no state change.
- Keep separate front/back slots so GL_POLYGON_MODE round-trips its two
  values (identical under a core context). The raster effect (VkPolygonMode
  + fillModeNonSolid) remains a backend follow-up; this is the state layer.

Tests: two RenderStateSanity round-trips; the glClampColor GL_FIXED_ONLY
acceptance assertion is mutation-verified (rejecting it fails the test).
Full SanityTest sweep green (29/29).

MobileGL/MG_State/GLState/Core.h

@@ -131,6 +131,11 @@ namespace MobileGL {
                 Float GetPointFadeThresholdSize() const;
                 void SetPointSpriteCoordOrigin(GLenum origin);
                 GLenum GetPointSpriteCoordOrigin() const;
+                void SetClampReadColor(GLenum clamp);
+                GLenum GetClampReadColor() const;
+                void SetPolygonMode(GLenum front, GLenum back);
+                GLenum GetPolygonModeFront() const;
+                GLenum GetPolygonModeBack() const;
                 void SetCapability(CapabilityInput cap, Bool enabled);
                 Bool IsCapabilityEnabled(CapabilityInput cap) const;
                 void SetCapabilityIndexed(CapabilityInput cap, Uint index, Bool enabled);