OpenGL: readback efb2ram with different strides at once

This is done with a pixel buffer object. We still have to stall the GPU, but
we only do it once per efb2ram call.
As the cpu can't access the vram, it has to queue a memcpy for the gpu and
wait for the gpu to finish this copy. We did this for every cache line which
is just stupid. Now we copy the complete texture into a pbo and readback this
at once. So we don't have to wait for lots of round-trip-times.