Fix keyboard event bug where mouse grab combination won't work if SDL_GetModState

[dyharlan]

I found this bug while trying to figure out why dingus won't work when compiled with MSVC (I still had to use Clang when compiling w/ vs2022).

Basically when the keyboard is in a specific state (NumLock, CapsLock, etc.) the code below:

if ( event.key.keysym.sym == SDLK_g && SDL_GetModState() == KMOD_LCTRL) {
    if (event.type == SDL_KEYUP) {
        toggle_mouse_grab(event.key);
    }
    return;
}

doesn't work because since if the keyboard is in a specific state, it returns the OR'd combination of said modifier, in this case CTRL, which has a value of 0x40, with the state of the keyboard.

So if the keyboard has all locks disabled, SDL_GetModState() returns the value of 0 | 0x40. But if NumLock is enabled, it returns the value of 0x1000 | 0x40 or 0x2000 | 0x40 if caps lock is enabled, meaning SDL_GetModState() == KMOD_LCTRL never gets evaluated to true if CTRL+G is pressed as it erroneously gets evaluated to 0x1040 == 0x40 if the combination is pressed if numlock is enabled, or 0x2040 == 0x40 if capslock is enabled.

The solution involves getting the mask of SDL_GetModState() and KMOD_LCTRL so that the expression SDL_GetModState() == KMOD_LCTRL can be checked properly regardless if the keyboard has any combination of caps lock/numlock enabled.

if (SDL_GetModState() & KMOD_LCTRL && event.key.keysym.sym == SDLK_g) {
    if (event.type == SDL_KEYUP) {
        toggle_mouse_grab(event.key);
    }
    return;
}

This was the reference I used to fix the issue:
https://stackoverflow.com/questions/48706762/sdl-2-how-to-check-for-no-modifiers-keyboard-input