#25 use GL_ARB_texture_non_power_of_two if available

[Eric Wasylishen]

Little patch for non-power-of-two support, to avoid blurring textures like the window in e4m3.

Doesn't mess with the padding to power-of-two that's done on non-tiling textures.

[Ozkan Sezer]

Shortened the boolean's name to gl_texture_NPOT and applied. Thanks.