Schedule blitting based on Metal Context state

In case of OpenGL we schedule blitting only when OpenGL context is not NULL in OGLRenderQueue.c->flushBuffer()

if (oglc != NULL) {
        RESET_PREVIOUS_OP();
        if (sync) {
            j2d_glFinish();
        } else {
            j2d_glFlush();
        }
        OGLSD_Flush(env);
    }

And it looks like OpenGL context is set in SET_SURFACES opcode.

In case of metal implementation we dont have any such check and we call scheduleBlitAllModifiedLayers() every-time we have flushBuffer call.

We need to verify whether such a difference is the reason we are facing performance degradation in Metal.