improve the graphic presets configurations

[mushis]

for example, in the fastest preset, the walls and floor become textureless and blueish. remove that, and use gl_textureless instead.

fastest:

• replace r_drawflat 1 with gl_textureless 1
• change r_floorcolor and r_wallcolor to something more faithful. suggesiton: 245 222 179 | 222 184 135
  add more please