For tiling, we will need to know the actual source size in addition to
the scaling factor that can be different. So store the scaling factors
in the scene command, as well as an offset where to start.
This is more accurate in case of clipping and rotation.
For rotation that doesn't matter (appart from the fact that the testing
can now be more strict)
But for clipping this prevent glitches with partial rendering where it
would seem like the image are moving a bit by a pixel when it is redrawn
with a different clip
