[TIMOB-6595] iOS: Ti.UI.Slider.thumbImage - custom image disappears when slider is being moved and physical pressure object (i.e., your thumb) is too close to the slider button

When a custom image is loaded into the Slider.thumbImage property, and the user moves the slider button, if their finger is too close to the slider button, the custom image temporarily disappears and is replaced by the default thumb image. This affects 1.8.0.1/1.7.5 iOS. Steps to Reproduce: 1. Run the attached project SliderTest (images included). 2. Use the slider button, drag it back and forth. Expected Result: The custom slider button image should be visible when the button is being moved. Actual Result: When the slider button is being moved, the custom image disappears and the default image takes it's place during the course of the movement. If the user moves their finger vertically away from the main slider while the slider button is being moved, the custom image reappears.