[Fixed] TEXTURES blending inconsistency with truecolor and paletted

[Gez]

Test map attached.

As reported here, the floor and ceiling in this test map are not represented consistently by ZDoom and GZDoom. However, as proved by the HELP graphic bound to F1, the blending performed by GZDoom is the same as the blending performed by ZDoom's DirectX display, meaning that the discrepancy does not come from GZDoom itself but from how the texture manager sends paletted blended textures vs. truecolor blended textures.

[randi]

Fixed in r3806.