[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.cpp

@@ -26,7 +26,12 @@ namespace MobileGL {
                 }
             } // namespace
 
-            RenderState::RenderState() {}
+            RenderState::RenderState() {
+                // The color writemask defaults to all-true for every draw buffer.
+                for (auto& mask : m_parameters.ColorMasks) {
+                    mask = BoolVec4(true, true, true, true);
+                }
+            }
 
             Uint RenderState::GetVersion() const {
                 return m_version;
@@ -437,14 +442,30 @@ namespace MobileGL {
 
             // -------------------- Color Mask --------------------
             void RenderState::SetColorMask(BoolVec4 mask) {
-                if (m_parameters.ColorMask == mask) return;
-
-                m_parameters.ColorMask = mask;
-                ++m_version;
+                // glColorMask broadcasts the same mask to every draw buffer.
+                Bool changed = false;
+                for (auto& slot : m_parameters.ColorMasks) {
+                    if (!(slot == mask)) {
+                        slot = mask;
+                        changed = true;
+                    }
+                }
+                if (changed) ++m_version;
             }
 
             BoolVec4 RenderState::GetColorMask() const {
-                return m_parameters.ColorMask;
+                // Non-indexed query reports draw buffer 0.
+                return m_parameters.ColorMasks[0];
+            }
+
+            void RenderState::SetColorMaskIndexed(Uint index, BoolVec4 mask) {
+                if (m_parameters.ColorMasks[index] == mask) return;
+                m_parameters.ColorMasks[index] = mask;
+                ++m_version;
+            }
+
+            BoolVec4 RenderState::GetColorMaskIndexed(Uint index) const {
+                return m_parameters.ColorMasks[index];
             }
 
             // -------------------- Clear State --------------------