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

Promote the color writemask to per-draw-buffer state and implement the
indexed glColorMaski entry point (previously a stub), plus its read-back
through glGetBooleani_v.

- RenderState: replace the single BoolVec4 ColorMask with an array of
  MAX_DRAW_BUFFERS masks, all initialized to true. SetColorMask now
  broadcasts to every draw buffer (glColorMask semantics); GetColorMask
  returns draw buffer 0. Add indexed set/get accessors + GLContext
  wrappers.
- glColorMaski sets only the addressed draw buffer; out-of-range index
  raises GL_INVALID_VALUE (buf is a GLuint, so no GL_INVALID_ENUM path),
  mirroring the indexed blend entry points' MAX_DRAW_BUFFERS bound.
- glGetBooleani_v(GL_COLOR_WRITEMASK, i) reports draw buffer i's four
  booleans; the non-indexed glGetBooleanv still reports draw buffer 0.
- Fix GLboolean coercion in the color-mask path: any nonzero value
  enables the component (was == GL_TRUE, which wrongly rejected e.g. 2).
- DirectGLES sync reads ColorMasks[0] (GLES core has only non-indexed
  glColorMask).

Tests: ColorMaskIndexedStoresAndReadsBack covers the per-buffer vs
broadcast semantics, buffer-0 read-back, out-of-range INVALID_VALUE, and
the GLboolean coercion (mutation-verified: == GL_TRUE fails it). Full
SanityTest sweep green (30/30).

MobileGL/MG_State/GLState/Core.cpp

@@ -493,6 +493,14 @@ namespace MobileGL::MG_State {
             return m_renderState.GetColorMask();
         }
 
+        void GLContext::SetColorMaskIndexed(Uint index, BoolVec4 mask) {
+            m_renderState.SetColorMaskIndexed(index, mask);
+        }
+
+        BoolVec4 GLContext::GetColorMaskIndexed(Uint index) const {
+            return m_renderState.GetColorMaskIndexed(index);
+        }
+
         void GLContext::SetClearColor(FloatVec4 color) {
             m_renderState.SetClearColor(color);
         }