change SkImage_Gpu to handle all filters (w/ and w/o gpu support

The result is that the set of "generic" imagefilters (e.g. SkColorFilterImageFilter) that use drawing commands to return their results will now stay in the same domain as their src (i.e. gpu-src --> gpu-dst).

ApplyFilterGM exercises this, and now asserts this same-domain invariant.

BUG=skia:4467

Review URL: https://codereview.chromium.org/1401053003
2 files changed