pixman-fast-path: Add fast path for "pad" type repeats

Similar in concept to fast_composite_tiled_repeat(), this breaks up any
unscaled composites, where source/mask areas outside the bitmap grid are
not clipped, into a series of simpler composites (either bitmap to bitmap
or solid to bitmap). These simpler composites are usually likely to match
existing fast path implementations, and so should benefit all platforms.

This produces some significant speedups for some cairo-perf-trace tests.
For example, timings on ARMv6 (using Siarhei's trimmed traces) are

Before:
[ # ]  backend                         test   min(s) median(s) stddev. count
[ # ]    image: pixman 0.29.3
[  0]    image         t-firefox-chalkboard   35.715   35.736   0.03%    6/6

After:
[ # ]  backend                         test   min(s) median(s) stddev. count
[ # ]    image: pixman 0.29.3
[  0]    image         t-firefox-chalkboard    9.254    9.261   0.15%    6/6

That's a speedup of 3.86x.

Also added a simple test program to check different repeat types.
lowlevel-blt-bench: Add in_n_8888 test
armv6: Add in_n_8888 fast path

lowlevel-blt-bench results:

    Before          After
    Mean   StdDev   Mean   StdDev  Confidence  Change
L1  18.8   0.1      63.9   0.9     100.0%      +239.0%
L2  16.0   0.4      58.5   1.3     100.0%      +265.8%
M   13.1   0.0      56.8   0.1     100.0%      +332.6%
HT  11.6   0.0      31.3   0.3     100.0%      +169.6%
VT  11.4   0.0      27.2   0.2     100.0%      +139.2%
R   11.0   0.1      28.2   0.2     100.0%      +156.1%
RT  6.8    0.1      12.9   0.2     100.0%      +89.0%