Add the ability to release and reacquire the high-power GPU, and to
respond to changes in the active GPU. In Chromium, the GPU process can
not access the WindowServer. An external process must inform ANGLE
that the active GPU has changed, and that ANGLE should switch its
internal context to the new GPU.

Incorporates a couple of functions from WebKit, used with permission,
to effect this GPU switch.

A follow-on change in Chromium which uses these new APIs will make the
existing dual-GPU tests pass with ANGLE and the passthrough command
decoder.

Carry forward Chromium's workaround of disabling GPU switching on
older MacBook Pros to ensure stability.

Document the process of adding new EGL extensions to ANGLE.

Bug: chromium:1091824
Change-Id: I499739156e851b493555d4d6e4aef87d8b97fa31
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2240638
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>