Fix for flats with transparent pixels on slopes

This fixes how R_DrawTiltedSplat_8 unintentionally allows the cyan pixels to NOT be considered "transparent" if after being remapped, depending on sector brightness and/or linedef type 606 colormaps, the result of remapping is not palette index 247 (the cyan we typically turn transparent). That is, the original colors from the source flat graphic are not checked, but instead the result of coloring the flat under the respective colormap is checked for "transparent" pixels. This is only a problem for the tilted splat drawing function, not the regular one for non-sloped planes with cyan-pixel-using flats.

I found out about this bug from the issues Ritz was having with sloped 255-alpha translucent FOFs using transparent flats and his custom COLORMAP lump (and later when he applied a linedef type 606 colormap to the FOF) for his custom map. Thankfully he has some workarounds, but this should fix the code-side issues that caused his problems in the first place.

I also fixed stuff with another splat drawing function that's not currently used atm (maybe it will be in the future, if splats themselves are ever enabled again? shrugs).