Improved lighting model

Walls and flats now use the same lighting model code and appear much closer to the original Doom style.

Your shaders were using different code to calculate light levels and thus had different light level scales causing lighting effects on flats to be inconsistent with their surrounding walls in the same sector. The pre-clamped `v_light * 2.0 / 31.0` scalar combined with the clamping for flats was causing more subtle lighting effects on the LSBs to be lost entirely, e.g. the glowing effect on the final teleport pad in doom1.wad e1m7. The scalar was reduced back to 1.0 / 31.0 as it should have been for both flats and walls, and clamping was removed in flats in favor of your superior wall-lighting code.

I also fixed flat texture lookup by inverting xz coords, e.g. see the final teleport pad's textures in doom1.wad e1m7. The goat skull texture appeared incorrectly without inverting the coords. This also fixes a few other floor/ceiling texture alignment issues.