Merge pull request #51 from nlordell/master

Added RENDER_TARGETS_RESET event
Added missing constant SDL_RENDER_TARGETS_RESET for SDL <= 2.0.1

Signed-off-by: Jacky Boen <aqiank@gmail.com>
Fixed some C functions not having return types

Signed-off-by: Jacky Boen <aqiank@gmail.com>