Directionality: Reduce unnecessary specificity of box shadows

[mikewheaton]

Prior to this change, the ms-box-shadow() mixin was using the LTR and RTL mixins for all shadows, even those that were identical for both directions.

[dir='ltr'] .vertical {
  box-shadow: 0 10px 5px 5px rgba(0, 0, 0, 0.5);
}
[dir='rtl'] .vertical {
  box-shadow: 0 10px 5px 5px rgba(0, 0, 0, 0.5);
}

It will now only return a single style, with less specificity, for vertical shadows:

.vertical {
  box-shadow: 0 10px 5px 5px rgba(0, 0, 0, 0.5);
}

[oengusmacinog-zz]

Nice logic!