internal/ui: optimize GPU usages when the screen doesn't have to be updated

This change skips rendering when 1) the screen is not cleared every frame
(`SetScreenClearedEveryFrame(false)`) and 2) Draw doesn't draw anything
onto the screen. The GPU usages decreased on some machines (e.g. GPU usage
was 10% with an empty Ebitengine project and became 2-3 % on a Windows
machine).

Updates #2341

gameforui.go

@@ -147,7 +147,7 @@ func (g *gameForUI) DrawOffscreen() error {
 	return nil
 }
 
-func (g *gameForUI) DrawScreen() {
+func (g *gameForUI) DrawFinalScreen() {
 	scale, offsetX, offsetY := g.ScreenScaleAndOffsets()
 	var geoM GeoM
 	geoM.Scale(scale, scale)