have shaders that need seed_shader call it themselves

This ought to make compose shader and our sprite blitter a bit more
efficient.  Compose shader can simply re-seed instead of saving the
xy values off to a buffer.  The sprite blitter doesn't need xy at all.

Change-Id: Ib4b3509288810f74a4c8e2978ce2ca14d8644980
Reviewed-on: https://skia-review.googlesource.com/18405
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed