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
