make inline version of premultiply, to speed up gradient creation.

We could speed-up again if we...
- respected kDither and only built 1/2 of the table for non-dither requests
- output simple params to the gpu rather than always a texture
- detected that we have no alpha, and then can skip premul per-entry



git-svn-id: http://skia.googlecode.com/svn/trunk@1772 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed