Initialize the supports partial update flag to false
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.
Edited by Vlad Zahorodnii