WindowScreen: Fix background being draw on top of screen

This was clearly wrong; just no one ever noticed because the vanilla content
drawn by `super.onDrawScreen` (e.g. vanilla buttons) is usually empty for
Elementa screens.
But now that I've seen it, I can't just keep it like that.

GitHub: #139

src/main/kotlin/gg/essential/elementa/WindowScreen.kt

@@ -48,11 +48,11 @@ abstract class WindowScreen @JvmOverloads constructor(
             afterInitialization()
         }
 
-        super.onDrawScreen(matrixStack, mouseX, mouseY, partialTicks)
-
         if (drawDefaultBackground)
             super.onDrawBackground(matrixStack, 0)
 
+        super.onDrawScreen(matrixStack, mouseX, mouseY, partialTicks)
+
         // Now, we need to hook up Elementa to this GuiScreen. In practice, Elementa
         // is not constrained to being used solely inside of a GuiScreen, all the programmer
         // needs to do is call the [Window] events when appropriate, whenever that may be.