renderer reset events

[mnabakker]

Great library, thank you for sharing this. SDL_FontCache does not seem to have an option for SDL event types SDL_RENDER_TARGETS_RESET(>= SDL 2.0.2) and SDL_RENDER_DEVICE_RESET(>= SDL 2.0.4). I added a function to handle both events https://github.com/mnabakker/SDL_FontCache/commit/6a269a734d8239a87d8420030e3c2e21486bed56 , but it makes no effort to optimize for SDL_RENDER_TARGETS_RESET.