[Pixman] More MIPS OVER fast paths (over_8888_n_8888, over_8888_n_0565, over_0565_n_0565, over_8888_8_8888, over_8888_8_0565, over_0565_8_0565, over_8888_8888 and over_8888_8888_8888) including OVER combiner.

[Nemanja Lukic]

Added optimizations for several OVER fast paths:
 - over_8888_n_8888
 - over_8888_n_0565
 - over_0565_n_0565
 - over_8888_8_8888
 - over_8888_8_0565
 - over_0565_8_0565
 - over_8888_8888
 - over_8888_8888_8888
Including OVER combiner.
Per previous code review:
 - Previously pushed single big commit is now divided into 4 smaller pieces.
 - Added OVER combiner.
Benchmark results (lowlevel-blt-bench) on Malta board (@1Ghz) are
included in the log messages.
Any comments to these patches are welcome.