libgui: Allow a device to force asynchronous composition

Part of the "hack all the things" series for devices with old
EGL blobs, do async mode when the requested usage includes
HW_RENDER, to avoid timing issues and stuck buffers.
Enable with the EGL_ALWAYS_ASYNC cflag

Change-Id: I5461ef1b2ac10ff36b7587eeec6979f09901c25b