[macOS] Fix missing text on loading error screens (#4532)

src/main/java/net/minecraftforge/fml/client/SplashProgress.java

@@ -651,6 +651,7 @@ public class SplashProgress
             checkThreadState();
             done = true;
             thread.join();
+            glFlush();        // process any remaining GL calls before releaseContext (prevents missing textures on mac)
             d.releaseContext();
             Display.getDrawable().makeCurrent();
             fontTexture.delete();