[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 and over_0565_8_0565).

[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
Benchmark results (lowlevel-blt-bench) on Malta board (@1Ghz) are
included in the log message.
Any comments to this patch are welcome.