expand SkShader's flag kConstInY to 16 and 32 variants, allowing a shader

expand SkShader's flag kConstInY to 16 and 32 variants, allowing a shader
(like gradients) to support predithering. If they do, then they would suppress
kConstInY16, since they no longer are const.

The blitters now check for each flag separately, so we don't have to give up
const-in-Y in the 32bit case, since in that mode we don't care about dithering.

git-svn-id: http://skia.googlecode.com/svn/trunk@339 2bbb7eff-a529-9590-31e7-b0007b416f81