[Pixman] More MIPS bilinear fast paths (src_8888_8888, src_8888_0565, src_0565_8888, src_0565_0565, over_8888_8888, add_8888_8888).

[Nemanja Lukic]

Added optimizations for several bilinear fast paths:
 - src_8888_8888
 - src_8888_0565
 - src_0565_8888
 - src_0565_0565
 - over_8888_8888
 - add_8888_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.
Any comments to this patch are welcome.