[Bug]: SDL2 driver goes to random resolution after fullscreen

[rubidium42]

Version of OpenTTD

13.0-RC1, Linux, SDL2 driver

Expected result

A reasonable SDL window size would be taken, e.g. the original full screen window size, of maybe 90% of that.

Actual result

The SDL window size is "undefined".

For me the result has been a super low and overly wide SDL window. That window is actually lower than the minimum height and wider than my screen.

Though also crash-worthy resolutions such as 22036 x 376383744 have occured.

Steps to reproduce

1. Start OpenTTD with SDL2 driver and go to the Game Options
2. Enable full screen
3. Disable full screen

Problem lies in sdl2_v.cpp's VideoDriver_SDL_Base::ToggleFullscreen where local variables w and h do not get set in case fullscreen = false, but at line 671 w and h are passed to some SDL function.
However, just flipping the expression of the if at line 654 doesn't seem completely right either. So, I hope someone has more inspiration how to fix this issue.

Actually found with the static code analysis that could be introduced by #10299.

[glx22]

VideoDriver_SDL_Base::ToggleFullscreen() code is strange, when switching to fullscreen it sets w and h but these are then write only. I think this function needs to be rewritten entirely.