SDL: wayland libdecor: windowed to fullscreen transition broken without SDL_WINDOW_RESIZABLE

Filing a new ticket for the remaining issues from https://github.com/libsdl-org/SDL/issues/4572#issuecomment-891461523.

Transitioning from windowed to fullscreen is broken in testgl2 and testvulkan when --resizable is not passed. This is a regression vs the same SDL revision built without libdecor support. testgles and testgles2 work but only because they explicitly pass SDL_WINDOW_RESIZABLE regardless of command-line arguments (and they break the same way as testgl2 when I comment it out).

I don’t think this is related to the Mutter full-screen issue fixed in https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1795, even though I am testing on a GNOME version that lacks the backport of that fix.

Steps to reproduce:

1. SDL_VIDEODRIVER=wayland ./testgl2 or SDL_VIDEODRIVER=wayland ./testvulkan
2. Press Ctrl+Enter to exit fullscreen

Results from testgl2:

Results from testvulkan:

Details: SDL a91ab883e970611cef0924df8aec7d264f93c8d1 libdecor-0.1.0-1.fc34 Fedora 34 GNOME 40.1.0