Improve performance of combine_over_u

Improve performance of combine_over_u

The generic C over_u combiner can be a lot faster with the
addition of special shortcuts for 0xFF and 0x00 alpha/mask
values. This is already implemented in C and SSE2 fast paths.

Profiling the run of cairo-perf-trace benchmarks with PIXMAN_DISABLE
environment variable set to "fast mmx sse2" on Intel Core i7:

=== before ===

37.32%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_u
21.37%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_no_repeat_8888
13.51%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_none_a8r8g8b8
 2.96%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] radial_compute_color
 2.74%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_a8
 2.71%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_x8r8g8b8
 2.17%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] _pixman_gradient_walker_pixel
 1.86%  cairo-perf-trac  libcairo.so.2.11200.0 [.] _cairo_tor_scan_converter_generate
 1.57%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_pad_a8r8g8b8
 0.97%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_in_reverse_u
 0.96%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_ca

=== after ===

28.79%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_no_repeat_8888
18.44%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_none_a8r8g8b8
15.54%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_u
 3.94%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] radial_compute_color
 3.69%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_a8
 3.69%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_x8r8g8b8
 2.94%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] _pixman_gradient_walker_pixel
 2.52%  cairo-perf-trac  libcairo.so.2.11200.0 [.] _cairo_tor_scan_converter_generate
 2.08%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_pad_a8r8g8b8
 1.31%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_in_reverse_u
 1.29%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_ca