Reimplement `SDL_BackendInit()` and `SDL_BackendShutdown()` for SDL 3.
Hide now unnecessary locking calls behind !USE_SDL3. To minimize code
duplication `SDL_Callback()` is hidden behind a wrapper function.
SDL_INIT_VIDEO includes SDL_INIT_EVENTS when initialized through
SDL_Init(), but not when shutdown to SDL_Quit(). Handle the shutdown
in the input subsystem. This closes some memory leaks in SDL.
With SDL 2 the refreshrate handling was rather limited by SDL 2 only
supporting integer refreshrates, making it impossible to represent the
common 59.95hz and other non-integer refreshrates. Quake II works around
this by assuming a refreshrate of 2hz higher than returned by SDL.
SDL 3 finally supports float refreshrates. Refactor the internal API to
also use floats.
This should be a no-op since the 'refreshrate * 1.02' logic stayes in
place for the time being. This can be reevaluated at a later time for
the SDL 3 build, but needs testing.
As a nice side effect fix the `vid_displayrefreshrate` cvar. It was
broken in both SDL 2 and 3 since it was always overwritten with the
actual display refresh rate.
In SDL 2 the first display was 0 and the error code -1. In SDL 3 this
changed to 1 as the first display and 0 for the error code. While at
it implement error handling and fallbacks for all cases.
SDL 3 has a new approach to fullscreen handling, which is way better
than the old fiddling with several window flags. In SDL 3:
* The SDL_FULLSCREEN_FLAG puts the window in fullscreen window mode.
* For exclusive fullscreen a fullscreen mode must be requested and set.
* Applying the mode to the window is asynchronous, the mode change may
occure at any later time. The window can be synchronized to force the
mode change.
This is - of course - totaly untested, since the client doesn't build
yet with SDL 3. There will likely be bugs and shortcomings.
And the code is still ugly, it can be refactored somewhat more.
Not many changes here. `make ref_gl1` now builds a library and it works
with the proof of concept port of the client.
SDL3 dropped support for hardware gamma, so `vid_gamma` will be ignored
until we can come up with something else. If we can, gamma without
shaders is hard to impossible :/
SDL3 TODOs will be marked with 'TODO SDL3:'.
This is hidden behind WITH_SDL3, which is disabled by default.
Additionally rename the SDL sources files in the client to represent the
SDL major version they are supporting. They will be forked for SDL 3.
That isn't optimal, because it forces us to support two variants of the
same code. However the changes between SDL 2 and 3 are too big to work
with #ifdef and something like function pointer magic is even more
confusing.
