[Bf-cycles] math and mix nodes, clamp patch (committed)

[Dalai Felinto]

Hi Brecht,
I know you are busy lately, so I committed two fixes straight to trunk:
rev. 50266  and 50271

I ran them through Lukas to get some review. But if you can check them
later it would be appreciated.
Feel free to do eventual changes yourself or to poke me with some directions.

Thanks,
Dalai