Request a high-priority EGL contexts

Summary:
This patch implements using EGL_IMG_context_priority to request
high-priority rendering contexts if the extension is available.

EGL_IMG_context_priority is currently used in this fashion by
e.g. Android's SurfaceFlinger (RenderEngine.cpp) and libweston
(gl-renderer.c) and seems promising given this widespread
acceptance.

Reviewers: #kwin, graesslin, romangg, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11572