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.

build-scripts/SDL_migration.cocci

@@ -2749,4 +2749,9 @@ expression e, n, v;
 expression w, i, s;
 @@
 - SDL_Vulkan_CreateSurface(w, i, s)
-+ SDL_Vulkan_CreateSurface(w, i, NULL, s)
++ SDL_Vulkan_CreateSurface(w, i, NULL, s)
+@@
+@@
+- SDL_RenderFlush
++ SDL_FlushRenderer
+  (...)