Revert "examples/shader: Rename ScreenSize -> ImageSize"

This reverts commit 3893f9cdbe.

Reason: The screen size and the image size can be different.

examples/shader/main.go

@@ -122,9 +122,8 @@ func (g *Game) Draw(screen *ebiten.Image) {
 	op.Uniforms = []interface{}{
 		float32(g.time) / 60,                // Time
 		[]float32{float32(cx), float32(cy)}, // Cursor
-		[]float32{float32(w), float32(h)},   // ImageSize
+		[]float32{float32(w), float32(h)},   // ScreenSize
 	}
-	// All the image sizes are (w, h).
 	op.Images[0] = gopherImage
 	op.Images[1] = normalImage
 	op.Images[2] = gopherBgImage