Improve EGL_ANGLE_power_preference on dual-GPU MacBook Pros. 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>