Added guess about G1 dpi.

Warning not tested.
sub0x67a934 turned into almost proper C code
Changed to memset to make code look a bit simpler
Skipping buggy code for now. Will be looked at some time later
Merge branch 'gfx_draw_sprite' of https://github.com/duncanspumpkin/OpenRCT2 into duncanspumpkin-gfx_draw_sprite
fix warnings
Merge branch 'duncanspumpkin-gfx_draw_sprite' into gfx_unstable