Xwayland: xwl_present_flip() can commit wl_surface when _XWAYLAND_ALLOW_COMMITS is set to 0

KWin and Mutter use _XWAYLAND_ALLOW_COMMITS to address visual glitches that can be observed during interactive resize.

However, it appears like _XWAYLAND_ALLOW_COMMITS doesn't work as advertised in all cases. Xwayland may actually commit the wl_surface in xwl_present_flip() after the _XWAYLAND_ALLOW_COMMITS property is set to 0. This makes interactive resizing choppy with applications such as Google Chrome in Plasma because the compositor waits for a wl_surface commit that had already been performed before it set the _XWAYLAND_ALLOW_COMMITS property back to 1.

Relevant discussion: #1743 (closed)