Accept vertical-first edge offsets in background-position

[jhaygood86]

Problem

The 3-/4-value background-position syntax positions each axis by an edge keyword plus an optional offset, combined with && — so the two axes may appear in either order (CSS Backgrounds 3 §3.6):

[ center | [ left | right ] <length-percentage>? ] && [ center | [ top | bottom ] <length-percentage>? ]

The horizontal-first forms work, but their vertical-first mirrors are rejected:

background-position: right 20px bottom 10px;  /* accepted */
background-position: bottom 10px right 20px;  /* rejected */
background-position: bottom 10px right;       /* rejected */
background-position: bottom right 20px;       /* rejected */

PointConverter has WithOrder(hi, vi, …) / WithOrder(hi, <lp>, vi, …) for the horizontal-first cases but no vertical-first equivalents.

Fix

Add the three mirrored WithOrder cases (vi hi <lp>, vi <lp> hi, vi <lp> hi <lp>) after the existing horizontal-first ones.

Two components on the same axis stay invalid — left right, top bottom, bottom 10px top, left 10px right 20px are all still rejected.

Tests

9 cases in PropertyTests/BackgroundProperty.cs: 5 vertical-first legal forms and 4 same-axis-twice illegal ones. The 5 legal cases fail on master; the illegal guards pass both ways. The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.

PointConverter also backs radial-gradient/conic-gradient positions, which gain the same symmetry.