analyze_extents: Fast path for non-transformed BITS images

analyze_extents: Fast path for non-transformed BITS images

Profiling various cairo traces showed that we were spending a lot of
time in analyze_extents and compute_sample_extents(). This was
especially bad for glyphs where all this computation was completely
unnecessary.

This patch adds a fast path for the case of non-transformed BITS
images. The result is approximately a 6% improvement on the
firefox-talos-gfx benchmark:

Before:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image            firefox-talos-gfx   13.797   13.848   0.20%    6/6

After:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image            firefox-talos-gfx   12.946   13.018   0.39%    6/6