Fix glitches with scaled windows for the XRender backend

Scale window contents and decorations togheter in order to
avoid 1px glitches caused by rounding errors;
This has the net benefit of scaling only one pixmap instead of two
for each window*clipper and should actually give some performance bonus

I've tested this for some time with no issues, but this commit
*might* cause regressions. In case, let me know.
CC