mesa: Reenable check for GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER_EXT

mesa: Reenable check for GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER_EXT

The check was disabled when FEATURE_OES_framebuffer_object was enabled,
since that used to mean we weren't implementing regular OpenGL semantics.
Now that we can compile in support for multiple APIs, change the #ifdef to
compile the check in when FEATURE_GL is enabled and enable the check for
contexts that implement OpenGL at runtime.