Reverted the order of the SDL_WarpMouseInWindow and SDL_GetMouseState
to its original sequence.
This avoids cursor warping to the center of screen on older SDL versions
It causes an additional SDL_MOUSEMOTION event when ungrabbing, but does
not cause any issues.
Move grab handling out of `nk_sdl_handle_event` to `nk_sdl_handle_grab`,
which is now called outside of the event loop.
This change makes the logic similar to the GLFW demos and fixes issues
with missed grab state changes and mouse cursor disappearing.
