Copy src in SkDrawLooper::computeFastBounds().

We do alias src and dst when building the BBH.  This meant we calculated
the wrong bounds in LayoutTests/ietestcenter/css3/text/textshadow-010.htm.

Not sure how best to test this inside Skia.  It may become moot after I
work through the API changes we were talking about today, but if not I'll
make sure it's tested somehow before I'm done with SkDrawLoopers.

BUG=skia:3186

Review URL: https://codereview.chromium.org/760023004
1 file changed