[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/RenderState/RenderState.h

@@ -230,8 +230,9 @@ namespace MobileGL {
         DepthTestFunc DepthFunc = DepthTestFunc::Less;
         Bool DepthMask = true;
 
-        // Color Mask
-        BoolVec4 ColorMask = BoolVec4(true, true, true, true);
+        // Color Mask. Per-draw-buffer state (glColorMaski); glColorMask broadcasts to all buffers.
+        // Every entry is initialized to all-true in RenderState's constructor.
+        Array<BoolVec4, MG_State::GLState::FramebufferObject::MAX_DRAW_BUFFERS> ColorMasks;
 
         // Clear State
         FloatVec4 ClearColor = FloatVec4(0.0f, 0.0f, 0.0f, 1.0f);
@@ -358,9 +359,13 @@ namespace MobileGL {
                                   StencilOperation depthPass);
                 const StencilFaceState& GetStencilState(StencilFace face) const;
 
-                // Color Mask
+                // Color Mask. SetColorMask broadcasts to every draw buffer and GetColorMask returns
+                // draw buffer 0; the indexed forms address a single draw buffer (glColorMaski). The
+                // caller is responsible for validating index against MAX_DRAW_BUFFERS.
                 void SetColorMask(BoolVec4 mask);
                 BoolVec4 GetColorMask() const;
+                void SetColorMaskIndexed(Uint index, BoolVec4 mask);
+                BoolVec4 GetColorMaskIndexed(Uint index) const;
 
                 // Clear State
                 void SetClearColor(FloatVec4 color);