i965: add software fallback for conformant 3D textures and GL_CLAMP

i965: add software fallback for conformant 3D textures and GL_CLAMP

The i965 hardware cannot do GL_CLAMP behavior on textures; an earlier
commit forced a software fallback if strict conformance was required
(i.e. the INTEL_STRICT_CONFORMANCE environment variable was set) and
2D textures were used, but it was somewhat flawed - it could trigger
the software fallback even if 2D textures weren't enabled, as long
as one texture unit was enabled.

This fixes that, and adds software fallback for GL_CLAMP behavior with
1D and 3D textures.

It also adds support for a particular setting of the INTEL_STRICT_CONFORMANCE
environment variable, which forces software fallbacks to be taken *all*
the time.  This is helpful with debugging.  The value is:
   export INTEL_STRICT_CONFORMANCE=2