(bug 45677) CSSJanus: support text-shadow and box-shadow flipping

We just need to negate the horizontal offset value in both of them.

This only supports *a single shadow* per element; multiple shadows
are not supported (only the first will be flipped).

Also, to make it possible:
* don't mangle 5+ consecutive numeric values in the
  'four_notation_quantity' rule
* support rgb(a) and hsl(a) colors in the 'color' rule

Change-Id: I148229558e1b9a0516e413ffe86007235c3c3ef8
Merge "(bug 45677) CSSJanus: support text-shadow and box-shadow flipping"