[Fix] (MG_Impl): report the draw-indirect binding and the buffer access state

Three pieces of queryable buffer state were missing, all of them read by the
KHR-GL40.draw_indirect basic-binding-* and basic-buffer-* cases:

- GL_DRAW_INDIRECT_BUFFER_BINDING had no case in glGetIntegerv, so it raised
  GL_INVALID_ENUM and left the caller's variable untouched (the test read back its own
  -9999 sentinel). GL_DISPATCH_INDIRECT_BUFFER_BINDING right next to it was already
  handled; this is the same two lines against BufferTarget::DrawIndirect. Because
  glGetBooleanv/glGetFloatv/glGetDoublev all widen from the integer path, one case
  fixes all four getters.
- GL_BUFFER_ACCESS answered 0 for an unmapped buffer. Its initial value is
  GL_READ_WRITE and glUnmapBuffer restores it (GL 4.6 core table 6.2); 0 is not a legal
  value of that state at all, and the test threw on the unrecognised enum.
- GL_BUFFER_ACCESS_FLAGS was not implemented, so it fell through to the invalid-pname
  arm. It is the MapBufferRange bitfield verbatim, which the mapping access flags
  already hold in normalised form - glMapBuffer's access enum is converted on the way
  in - so it converts straight back out, and reads zero while unmapped.

MobileGL/MG_Util/Converters/MGToGL/BufferEnumConverter.cpp

@@ -73,7 +73,7 @@ namespace MobileGL {
             }
         }
 
-        GLbitfield ConvertBufferMappingAccessToGLEnum(BufferMappingAccessBit access) {
+        GLbitfield ConvertBufferMappingAccessToGLEnum(Flags<BufferMappingAccessBit> access) {
             GLbitfield result = 0;
             if (access & BufferMappingAccessBit::Read) result |= GL_MAP_READ_BIT;
             if (access & BufferMappingAccessBit::Write) result |= GL_MAP_WRITE_BIT;

MobileGL/MG_Util/Converters/MGToGL/BufferEnumConverter.h

@@ -14,6 +14,6 @@ namespace MobileGL {
     namespace MG_Util {
         GLenum ConvertBufferTargetToGLEnum(BufferTarget bufferTarget);
         GLenum ConvertBufferUsageToGLEnum(BufferUsage usage);
-        GLbitfield ConvertBufferMappingAccessToGLEnum(BufferMappingAccessBit access);
+        GLbitfield ConvertBufferMappingAccessToGLEnum(Flags<BufferMappingAccessBit> access);
     } // namespace MG_Util
 } // namespace MobileGL