SCUMM: Fix Loom (and some other graphics regressions).

These are regressions from c05cb7f3bbcf4d64d4a938e0eb42065d8f3d3038. They were
caused by VirtualScreen::getPixels differing from Surface::getBasePtr and I
accidently used the former in some cases in the conversion.

I also fixed a bug in debugger.cpp which exchanged x and y.
SCUMM: Add comment about getPixels uses in BaseCostumeRenderer.