st3m_gfx: fix direct ctx variants

Using busy-polling to wait for blit to be done, using a mutex here instead did not work as intended. The direct modes are good for debugging and currently without a graphics task that floats between cores can be used in scenarios where cpu load for audio is high enough to starve graphics. The direct modes also are good in that you can immediately read rendered pixels back from framebuffers after issuing drawing commands and do not need to wait for the next frame.