SkJumper: skip null contexts

This makes stages that don't use a context pointer look a little
cleaner, especially on ARM.  No interesting speed difference on x86.

What do you think?

Change-Id: I445472be2aa8a7c3bc8cba443fa477a3628118ba
Reviewed-on: https://skia-review.googlesource.com/9155
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed