render: SDL_RenderPresent now behaves when a render target is currently set.

It'll temporarily unbind it, draw letterboxing if necessary, present, and
then rebind the previous render target.

src/render/SDL_render.c

@@ -4949,9 +4949,9 @@ bool SDL_RenderPresent(SDL_Renderer *renderer)
 
     CHECK_RENDERER_MAGIC(renderer, false);
 
-    // !!! FIXME: fail if using a render target? Or just implicitly set the render target to NULL now?
-    if (renderer->view != &renderer->main_view) {
-        return SDL_SetError("Cannot present a render target");
+    SDL_Texture *target = renderer->target;
+    if (target) {
+        SDL_SetRenderTarget(renderer, NULL);
     }
 
     SDL_RenderLogicalPresentation(renderer);
@@ -4972,6 +4972,10 @@ bool SDL_RenderPresent(SDL_Renderer *renderer)
         presented = false;
     }
 
+    if (target) {
+        SDL_SetRenderTarget(renderer, target);
+    }
+
     if (renderer->simulate_vsync ||
         (!presented && renderer->wanted_vsync)) {
         SDL_SimulateRenderVSync(renderer);