Move implementation of SkFilterShader into its source file.

Move implementation of SkFilterShader into its source file.

Since SkFilterShader has its own header file, makes more sense to have a
separate source file to implement it rather than placing it in a "random"
SkColorFilter.cpp file.

R=robertphillips@google.com

Committed: https://code.google.com/p/skia/source/detail?r=10101

[robertphillips, 2013-07-15 23:58:00]

lgtm

[tfarina, 2013-07-16 13:05:35]

Committed patchset #1 manually as r10101 (presubmit successful).