backends/wayland: Try to reallocate output layer data on beginFrame()

Mesa's implementation of wl_egl_window_resize() is pretty smart. It
avoids immediately resizing the buffers. On the other hand, its API
doesn't enforce that behavior and other implementations of wayland-egl
may choose to use a different strategy.

This change moves wl_egl_window_resize() to beginFrame() to make buffer
resizing handling more nicer. Also, for the record, the drm backend
works in similar fashion.