Fix GL context switching in a case of vrend using async fences and external EGL display
QEMU has own EGL display surface that it shares with virglrenderer. After switching QEMU to use async fences, the vrend's GL fence waiting thread fails to switch EGL context because EGL display surface is used by QEMU in a different EGL thread. Create and use own surfaceless EGL context when using external display surface, instead of using the external EGL context, fixing the context switching problem for QEMU.