Keymod from held modifier key is lost when setting/clearing full-screen.

[pshaughn]

If I start holding a modifier key such as Shift or Alt before a call to SDL_SetWindowFullscreen, then I keep holding it down and press some keys after that call resizes the window, the events from the new key presses don't include the modifier from the held key. Releasing the modifier key and pressing it again works, and other modifier keys that I start holding after the window size change work. SetWindowPosition and SetWindowSize don't do this, just Fullscreen.

(Platform: I'm using Wine to run an i686 executable on 64-bit Linux; SDL is at commit 5c020b9.)