render: Replaced SDL_RenderFlush with SDL_FlushRenderer.

This uses the same `SDL_VerbNoun` format as the rest of SDL3, and also
adds stronger effort to invalidate cached state in the backend, so cooperation
improves with apps that are using lowlevel rendering APIs directly.

Fixes #367.

src/render/SDL_sysrender.h

@@ -179,6 +179,7 @@ struct SDL_Renderer
                          int num_vertices, const void *indices, int num_indices, int size_indices,
                          float scale_x, float scale_y);
 
+    void (*InvalidateCachedState)(SDL_Renderer *renderer);
     int (*RunCommandQueue)(SDL_Renderer *renderer, SDL_RenderCommand *cmd, void *vertices, size_t vertsize);
     int (*UpdateTexture)(SDL_Renderer *renderer, SDL_Texture *texture,
                          const SDL_Rect *rect, const void *pixels,