Use 'auto' instead of 'initial' in CSS

'initial' is not supported by all browsers (including all versions of
Internet Explorer and Opera 12 [1]), which causes visual glitches [2].

As the initial value for 'left' is 'auto' [3], this should cause no
visual changes in the browsers that supported the previous version.

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/initial
[2] http://i.imgur.com/0Ze8lon.png
[3] http://www.w3.org/TR/CSS21/visuren.html#position-props

Change-Id: I8d661436f1106ca92b8b492c3fb76ac6976d80fa

resources/ext.popups.core.less

@@ -142,14 +142,14 @@
 	&.flipped_x.mwe-popups-no-image-tri {
 		&:after {
 			/* @noflip */
-			left: initial;
+			left: auto;
 			/* @noflip */
 			right: 11px;
 		}
 
 		&:before {
 			/* @noflip */
-			left: initial;
+			left: auto;
 			/* @noflip */
 			right: 10px;
 		}