mesa,st/mesa: implement GL_RGB565 from ARB_ES2_compatibility

This was not implemented, because the spec was changed just recently.

Everything has been in place already.

Gallium has PIPE_FORMAT_B5G6R5_UNORM, while Mesa has MESA_FORMAT_RGB565.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/fbobject.c

@@ -1296,6 +1296,9 @@ _mesa_base_fbo_format(struct gl_context *ctx, GLenum internalFormat)
 
    case GL_RGB10_A2UI:
       return ctx->Extensions.ARB_texture_rgb10_a2ui ? GL_RGBA : 0;
+
+   case GL_RGB565:
+      return ctx->Extensions.ARB_ES2_compatibility ? GL_RGB : 0;
    default:
       return 0;
    }