rename (x,y) to (dx,dy)

Today (x,y) are the integer coordinates of the first destination pixel
we're working on.  By renaming them (dx,dy), we free up the names (x,y)
for working (i.e. _source_) x and y.

Until now we've generally just been continuing to call those (r,g), but
in the _lowp code that won't be possible (r+g hold x together, b+a y)
but we'll have the ability to just give them proper names x and y.

Change-Id: Id5faa09c4406116df5df7494efc6cb23659e9a2f
Reviewed-on: https://skia-review.googlesource.com/60820
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

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