Fix flicker when changing backend on Mac viewer

Tie the SDL GL context to the Window, not the context. Deleting the context
causes the window to render a frame of black - this keeps the context alive
along with the window, so that never happens.

Change-Id: Id4df18a6f2fe09f617ec2ff1809d000f18f547ba
Reviewed-on: https://skia-review.googlesource.com/107941
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed