Cache the results of get_pixel_formats_to_try

Choosing pixel formats is quite slow (depending on driver?). We were
doing this once per context, and it added up. On my desktop windows
machine, this saves another 7 seconds in `dm --config gl --src gm`.

Actual times:
  37s -> 30s (not writing PNGs)
  47s -> 39.5s (writing PNGs)

We always called this with MSAA sample count set to zero, so I cleaned
up the code to make that clearer. Also included a comment about the
theoretical risk, although I think that outside of a multi-GPU system,
we're fine.

Bug: skia:
Change-Id: I50927ebfaf6fe8d88a63674427fbf9e06e4ab059
Reviewed-on: https://skia-review.googlesource.com/35763
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

0 files changed, 0 insertions, 0 deletions