examples/cursor: Change the background colors

On Windows, cursor colors are determined based on the background
color, and if the background color is gray, the cursor might be
invisible. This change adjust the color so that the cursor should
be visible in any cases.

Closes #1583

examples/cursor/main.go

@@ -79,11 +79,16 @@ func main() {
 		gridColors: map[image.Rectangle]color.Color{},
 	}
 	for rect, c := range g.grids {
-		a := byte(0x40)
-		if c%2 == 0 {
-			a += 0x40
+		clr := color.RGBA{0x40, 0x40, 0x40, 0xff}
+		switch c % 3 {
+		case 0:
+			clr.R = 0x80
+		case 1:
+			clr.G = 0x80
+		case 2:
+			clr.B = 0x80
 		}
-		g.gridColors[rect] = color.Alpha{a}
+		g.gridColors[rect] = clr
 	}
 
 	ebiten.SetWindowSize(screenWidth, screenHeight)