OpenGL extension GL_EXT_texture_compression_s3tc should be optional

[JayFoxRox]

There's no report of this being a problem for any GPU driver (yet).
However, while this extension is probably supported anyway, it wouldn't be bad to make it entirely optional.

We'll probably need our own DXT decoder for some use-cases anyway:

• Signed textures (once NV_PGRAPH_TEXFILTER_SIGNED not supported #75 is solved, there'll probably be a new issue)
• DXT volume textures (see DXT compression in volume textures (Assertion `f.gl_format != 0' failed) #118 )
• Small mipmap levels (no issue, but (poor) explanation in code)

We should keep the existing OpenGL texture format as optimization, but a software fallback can easily be integrated into our texture conversion routine.