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