Add missing optimization to software npo2 sloped floor sprites

Adds an optimization from non-power-of-two texture slope planes to sloped npo2 floor sprites.

Before:

After:

Test map and script:

simple_box.wad sw-splat-npo2-slope-opt.lua

Test build: (based on next so be careful with your save files)

srb2win_sw-splat-npo2-slope-opt.exe