Window does not automatically resize on screen resolution change

[theofficialgman]

SDL2 allows for automatic changing of screenresolution and updating the viewport. Currently, when the screen resolution is changed while emulationstation is open or during a game and exiting back to emulationstation, the rendered emulation station view is not changed (so the image is either too large for the screen or too small).

A combination of SDL_WINDOW_RESIZABLE and SDL_WINDOW_FULLSCREEN_DESKTOP should allow this to occur. Retroarch can be referenced for fixing this issue as it handles resolution changes while running in game gracefully.