With 870679e4, if the partial update
extension is unsupported, setSupportsPartialUpdate() won't be called.

The problem is that it may leave OpenGLBackend::supportsPartialUpdate()
uninitialized, which can lead to a crash if an OpenGL render backend
tries to call eglSetDamageRegionKHR() and EGL_KHR_partial_update is
unsupported.