Fix font lost parentAlpha in HighlightTextArea

[fgnm]

In HighlightTextArea font color is overwritten by chunk color. This color doesn't know anything about parent alpha and thus all the work done here in VisTextField is useless.
This commit saves original parent alpha value and restore it after chunk color set.

This fix prevent some ugly animations when HighlightTextArea is placed for example in a Dialog that fades in and out.

You can test with TestHighlightTextArea too, just change the alpha of the window.

Thanks

[fgnm]

I've added another commit because I forgot that chunk color might have its own alpha, so this should be preserved as well

[kotcrab]

Good catch, thanks.