[Pixman] Several MIPS fast paths.

[Nemanja Lukic]

Added optimizations for several:
  - SRC fast paths:
    - src_n_8_8888
    - src_n_8_8
  - OVER fast paths:
    - over_n_0565
    - over_n_8888
  - OVER nearest neigbor scaling fast paths:
    - over_8888_8_0565
    - over_0565_8_0565
Benchmark results (lowlevel-blt-bench) on Malta board (@1Ghz) are
included in the log messages.
Per previous code review:
 - Fixed performance regression for L1 and L2 for over_n_8888 routine.
Any comments to these patches are welcome.