Image filters: simplify filterInputGPU().

Remove a call to canFilterImageGPU() / filterImageGPU() from
filterInputGPU(). There's no reason to do this, since
the subsequent filterImage() call will do it for us anyway.
And this call actually defeats caching (as demonstrated by
the attached bench).

BUG=skia:

Review URL: https://codereview.chromium.org/1411013004
2 files changed