[Feat] (MG_State, MG_Impl): transform feedback state, validation and reflection

First stage of GL 3.0 transform feedback: glTransformFeedbackVaryings /
glGetTransformFeedbackVarying / glBeginTransformFeedback /
glEndTransformFeedback were unimplemented stubs. This adds

- per-program capture state: requested varyings apply on the next link and
  resolve against the last vertex-processing stage's linker objects (with
  gl_Position/gl_PointSize handled as builtins), failing the link on
  unknown or duplicate names or exceeded interleaved/separate limits, with
  offsets and strides computed per GL rules;
- context Begin/End state with the GL 3.3 error semantics: invalid
  primitive modes, redundant Begin/End, missing program or capture-buffer
  bindings, primitive-mode compatibility at draw time, and the
  while-active prohibitions on rebinding capture buffers, switching
  programs, and relinking the captured program;
- GetProgramiv TRANSFORM_FEEDBACK_* queries and a 4-slot bound on indexed
  GL_TRANSFORM_FEEDBACK_BUFFER binding points.

KHR-GL33.transform_feedback api_errors/linking_errors/get_xfb_varying now
pass; GPU-side capture is the remaining stage.

MobileGL/MG_Impl/GLImpl/Buffer/Validators.cpp

@@ -60,6 +60,11 @@ namespace MobileGL::MG_Impl::GLImpl::BufferImpl {
                 MG_Backend::pActiveBackendObject->GetDynamicParameters().MaxShaderStorageBufferBindings;
             pointCount = std::min(pointCount, static_cast<SizeT>(std::max(backendCount, 0)));
         }
+        if (target == BufferTarget::TransformFeedback) {
+            // GL_MAX_TRANSFORM_FEEDBACK_SEPARATE_ATTRIBS bounds the indexed capture
+            // binding points in GL 3.3 (no ARB_transform_feedback3).
+            pointCount = std::min<SizeT>(pointCount, 4);
+        }
 
         if (index < pointCount) {
             return true;