ColorButton: fixed double style alpha multiply. Remove call to GetColorU32() in RenderColorRectWithAlphaCheckerboard() for consistency. (#9511)

The main fix is the change from GetColorU32() to ColorConvertFloat4ToU32() in calling code.
The change from GetColorU32() to manual multiply in RenderColorRectWithAlphaCheckerboard() is merely for consistency with other ::Render calls.