This means we don't have to enable ARB_draw_buffers explicitly.

If the needed constants are available, we can support all vs_2_0 and ps_2_0
requirements with the plain ARB extensions. We cannot however, run SM 2.0a or
SM 2.0b.

ps 1.x constants are clamped to [-1;1], constants in >= 2.0 pshaders
are not. This means we have to reload constants when switching between
those shader types in ARB. In GLSL this is not a concern because
constants are tied to program objects and are reloaded on a shader
change anyway.

This patch tries to find a free texture coordinate to load up to 4 clip
coordinates into the pixel shader, and uses KIL to throw away fragments
that are cut by a clipplane. If no free texture coordinate is found,
clipping is not done. If more than 4 clipplanes are used, only the first
4 are actually enabled. That should be pretty rare though.