Use const pointers for source data.

Our uniform data could be in a const buffer, so we should try to
be const-correct where we can.

Change-Id: Iecc9ae152e1153de464407d856c434b713f447d2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/620227
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed