internal/devicescale: bug fix: glfw.Monitor.GetContentScale might return 0

Retrying to call GetContentScale solved this.

Closes #2051

internal/devicescale/impl_desktop.go

@@ -35,6 +35,13 @@ func monitorAt(x, y int) *glfw.Monitor {
 }
 
 func impl(x, y int) float64 {
-	sx, _ := monitorAt(x, y).GetContentScale()
-	return float64(sx)
+	// Keep calling GetContentScale until the returned scale is 0 (#2051).
+	// Retry this at most 5 times to avoid an inifinite loop.
+	for i := 0; i < 5; i++ {
+		sx, _ := monitorAt(x, y).GetContentScale()
+		if sx != 0 {
+			return float64(sx)
+		}
+	}
+	return 1
 }