Fix SkSwizzler bug

Now, that we are subsetting, fX0 is not necessarily less than
fSrcWidth (since fSrcWidth is really the subset width).

Ex: We may want a 10 pixel subset starting twenty pixels from the
left edge.  In that case, fX0=20 and fSrcWidth=10.

Let's rename the width as fSubsetWidth to avoid confusion
and remove the check.

BUG=skia:

Review URL: https://codereview.chromium.org/1407603003
5 files changed