[Pixman] MIPS bilinear fast paths (src_8888_8_8888, src_8888_8_0565, src_0565_8_x888, src_0565_8_0565, add_8888_8_8888).

[Nemanja Lukic]

Added optimizations for several bilinear fast paths:
 - src_8888_8_8888
 - src_8888_8_0565
 - src_0565_8_x888
 - src_0565_8_0565
 - add_8888_8_8888
Benchmark results (using tweaked version of the lowlevel-blt-bench which does 
bilinear scaling using almost identity matrix) on Malta board (@1Ghz) are 
included in the log message.
These routines still use old BILINEAR_INTERPOLATE_SINGLE_PIXEL macro which has 
potential to be more efficient (as pointed out in previous code review). This 
improvement will be part of the future MIPS bilinear fast-path commits.
Any comments to this patch are welcome.