Also merge CreateSystemTimer() with WIN16_CreateSystemTimer() and
rename it to CreateSystemTimer16() for consistency.

comctl32: Use user32 control names from public header constants instead of defining over and over again.

comctl32/monthcal: Cleanup top drawing helper namespace, reduce parameter count in prev/next draw helpers.

add_sRGB_correction and the generate_pshader function generate the same MOV.
There's no need to do it in two places.