sdl: events.go: Add missing SDL_DROPTEXT, SDL_DROPBEGIN, and SDL_DROPCOMPLETE constants
sdl: events.go: Add missing SDL_DROPTEXT, SDL_DROPBEGIN, and SDL_DROPCOMPLETE constants

Signed-off-by: Lilis Iskandar <f9acb790f2c75b127492316321ece0121b8eed9b@veand.co>

veandco/go-sdl2,veandco/go-sdl2