internal/ui: skip rendering onto the final framebuffer only for OpenGL

This was reverted at a049acd but now
I've confirmed this is not problematic only with OpenGL.

Updates #2341
Updates #2342

internal/ui/context.go

@@ -198,19 +198,20 @@ func (c *context) drawGame(graphicsDriver graphicsdriver.Graphics, forceDraw boo
 		c.skipCount = 0
 	}
 
-	// TODO: If the offscreen is not updated and the framebuffers don't have to be updated, skip rendering to save GPU power
-	// (#2341, #2342). The condition would be `c.skipCount < maxSkipCount`.
-	// However, Metal (and maybe DirectX) cannot vsync without swapping the buffer by rendering the screen framebuffer (#2520).
-	if graphicsDriver.NeedsClearingScreen() {
-		// This clear is needed for fullscreen mode or some mobile platforms (#622).
-		c.screen.clear()
-	}
+	// TODO: Metal (and maybe DirectX) cannot vsync without swapping the buffer by rendering the screen framebuffer (#2520).
+	// Implement this skipping appropriately for Metal and DirectX.
+	if c.skipCount < maxSkipCount && graphicsDriver.IsGL() {
+		if graphicsDriver.NeedsClearingScreen() {
+			// This clear is needed for fullscreen mode or some mobile platforms (#622).
+			c.screen.clear()
+		}
 
-	c.game.DrawFinalScreen(c.screenScaleAndOffsets())
+		c.game.DrawFinalScreen(c.screenScaleAndOffsets())
 
-	// The final screen is never used as the rendering source.
-	// Flush its buffer here just in case.
-	c.screen.flushBufferIfNeeded()
+		// The final screen is never used as the rendering source.
+		// Flush its buffer here just in case.
+		c.screen.flushBufferIfNeeded()
+	}
 
 	return nil
 }