[SparkSkins] Spark NumericStepper doesnt play nice w/ contentBackgroundAlpha of 0%

Found in 4.0.0.10237

Steps to reproduce:
1. Compile and run attached MXML file.
2. Focus in the two NumericStepper controls and click the up/down buttons.

Actual Results:
Ghosting of text in Spark NumericStepper.

Expected Results:

Workaround (if any):
n/a

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/halo">

<s:Panel x="20" y="20" initialize="event.currentTarget.title = mx_internal::VERSION;">
<s:Rect width="100%" height="100%">
<s:fill>
<s:LinearGradient rotation="45">
<s:GradientEntry color="red" />
<s:GradientEntry color="yellow" />
<s:GradientEntry color="red" />
</s:LinearGradient>
</s:fill>
</s:Rect>
<s:VGroup>
<s:NumericStepper contentBackgroundAlpha="0" fontSize="34" />
<mx:NumericStepper contentBackgroundAlpha="0" fontSize="34" />
<s:TextInput text="Spark" contentBackgroundAlpha="0" fontSize="34" widthInChars="5" />
<mx:TextInput text="Halo/MX" contentBackgroundAlpha="0" fontSize="34" />
</s:VGroup>
</s:Panel>

</s:Application>