Register some flattenable factories.

Now, SkComposeShaders, SkDiscreteMapper, SkCosineMapper,
Line2DPathEffect, and SkTransparentShader can be flattened
and unflattened. This makes more of our samples work when
using a pipe.

Review URL: https://codereview.appspot.com/6338064

git-svn-id: http://skia.googlecode.com/svn/trunk@4334 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed