[Pixman] MIPS over_n_8_8888, over_n_8_0565 and bilinear over_8888_8_8888 fast paths.

[Nemanja Lukic]

Added optimizations for over_n_8_8888, over_n_8_0565 and bilinear over_8888_8_8888 routines.
Benchmark results (lowlevel-blt-bench and cairo-perf-trace) on Malta board (@1Ghz) are included in the log message.
Per previous code review:
  - When (srca == 0xff && mask == 0xff) 8888 to 0565 conversion is done only once.
Any comments to these patches are welcome.