Added SDL_RenderViewportSet() to tell whether the viewport was previously set to a specific rectangle.

Fixes https://github.com/libsdl-org/SDL/issues/9029

src/dynapi/SDL_dynapi.sym

@@ -971,6 +971,7 @@ SDL3_0.0.0 {
     SDL_GetRenderColorScale;
     SDL_RenderGeometryRawFloat;
     SDL_SetWindowShape;
+    SDL_RenderViewportSet;
     # extra symbols go here (don't modify this line)
   local: *;
 };