llvmpipe: fix out-of-bounds texture sampling

llvmpipe: fix out-of-bounds texture sampling

If we're using a wrap mode in which border color sampling is possible
it means that texcoords may be outside of the texture image bounds.
Fetching the texel may result in a segfault.

Use the 'use_border' variable to catch such texcoords and replace
the texel offset with zero (which will be in bounds).

Fixes segfault in Lightsmark demo, fd.o bug 27877.