Fix resize handling when enabling and disabling full-screen mode.

Fix resize handling when enabling and disabling full-screen mode.

I'm not sure whether I broke this in the recent revamp or whether it
was always broken, but: transitions in and out of full-screen mode
work by first maximising or restoring the window, which triggers a
WM_SIZE, whose handler then fiddles with the window style to disable
or re-enable all the furniture, which in turn triggers a recursive
WM_SIZE. The trouble is, after returning from the handler for the
inner WM_SIZE, the rest of the outer handler runs, and its client area
size is now out of date.

So I've added a flag which is set when a resize is handled 'properly',
so that after returning from the inner WM_SIZE handler the outer one
knows not to try to redo badly work that's already been done well.

[originally from svn r9056]