Text patch (NY00123)

Two main engine-side changes:
- id_vh* and id_vl* have new methods, letting you "xor" graphics onto the virtual screen. In the SDL 1.2 case the screen has XRGB format, so this may be a bit of a problem if the palette is changed. But for now it doesn't happen, so we have the current code (and of course SDL 2.0).
- Three functions in id_vh.c, including VH_MeasurePropString, have been modified to pick the correct font chunk (offset of 3 added).
- Only US_GetPrintFont, US_GetPrintColour and the respective setters are used. (Would be much simpler to access the variables directly, although less clean in a way, I agree.)