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).

Second attempt. First, reverted one: I14822955.

Bug: 45677
Change-Id: I97ee7431e1a5acb35d594076a88a0f9acf290402
Merge "CSSJanus: Support text-shadow and box-shadow flipping"