Define and expose EGL_ANGLE_power_preference extension. Allows application to select the integrated or discrete GPU on dual-GPU macOS systems. Tested by modifying the example program at: https://github.com/grorg/ANGLEIOSurfaceTest and verifying that both integrated and discrete GPUs can be selected. (The changes to that program will be upstreamed once some build issues are resolved.) Bug: 2813 Change-Id: Ibeb17778512896456d220e9bc4cf8f222aa57057 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1570081 Commit-Queue: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>