Add SDL_SetModalLoopCallback, SDL_SetModalLoopResizeCallback and SDL_SetModalLoopMoveCallback functions

[RT2Code]

Add three functions to set callbacks to be called during a window modal loop, allowing the user to keep his app running when it happens.

The message handling require more work to ensure it is robust enough and that there's no corner case left out.

Fix #1059

[RT2Code]

I think it is ready to be reviewed.

[MyreMylar]

Looks like this needs to be updated for 2.0.22 now since it didn't make it in for 2.0.18 or 2.0.20.

[RT2Code]

Done. 😉

[sezero]

Can you please rebase to current master squash into a single commit?

@slouken, @icculus: yours to review.

[hstormo]

Perhaps there should also be a callback specifically for WM_MOVE. The window enters the same modal loop when being dragged, and some applications may want to update the window contents based on the window's position on the screen as well as its size.

[RT2Code]

@sezero Sure ;)

@hstormo I agree with you. I added the SDL_SetModalLoopMoveCallback function.

[tsedan]

Sorry, just checking if there's something blocking this? Would love to see this issue closed after nearly nine years 😅

[slouken]

This is interesting, but ideally we would find a way to handle this so you don't need specialized callbacks to deal with move and resize.

Here is another thread with more info on this that I would like to research for SDL 3.0:
#1059

[RT2Code]

Fair enough, if you can implement this without the need to use callbacks, it would be perfect indeed. 😃

Should we close this PR?

[slouken]

No, let's leave it open because it might end up being the best solution.

[slouken]

I've added a solution that dovetails nicely with the new main callbacks in SDL 3.0.

Thanks for the example and all the feedback!