Set up eGPU/discrete support for MacOS.

Pulled out of https://skia-review.googlesource.com/c/skia/+/271319.

For Metal, will default to an eGPU or discrete GPU if one is available.
For GL, will attempt to use a Radeon eGPU, and will fallback
if one can't be found.

Change-Id: I0a1efb3afca612ac75be56f633d811dda68f9d10
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277516
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
6 files changed