Fix comment about BILINEAR_INTERPOLATION_BITS to say < 8 rather than <= 8

Since a4c79d695d52c94647b1aff7 the constant
BILINEAR_INTERPOLATION_BITS must be strictly less than 8, so fix the
comment to say this, and also add a COMPILE_TIME_ASSERT in the
bilinear fetcher in pixman-fast-path.c