Fixed float to int conversion warning, which was a legitimate bug.
Added a relative mouse mode that uses mouse warping instead of raw input.
To enable this, set the environment variable SDL_MOUSE_RELATIVE_MODE_WARP to "1"

When mouse relative mode is disabled, put the cursor back where the application expects it to be, instead of where it was when relative mode was enabled.
Setting the mouse in relative mode implies grabbing the mouse.
This fixes getting mouse button events in raw input relative mode on X11.