[css-masking] Make interpolation of mask-border-* consistent with border-image-*

Fixes #529

Author: nt1m

css-masking-1/Overview.bs

@@ -1031,7 +1031,7 @@ Inherited: no
 Percentages: refer to size of the <a>mask border image</a>
 Computed value: as specified
 Media: visual
-Animation type: discrete
+Animation type: by computed value
 </pre>
 
 This property specifies inward offsets from the top, right, bottom, and left edges of the <a>mask border image</a>, dividing it into nine regions: four corners, four edges and a middle. The middle image part is discarded and treated as fully opaque white (the content covered by the middle part is not masked and shines through) unless the <dfn dfn-for=mask-border-slice dfn-type=value>fill</dfn> keyword is present.
@@ -1050,7 +1050,7 @@ Inherited: no
 Percentages: relative to width/height of the <a>mask border image area</a>
 Computed value: all <<length>>s made absolute, otherwise as specified
 Media: visual
-Animation type: discrete
+Animation type: by computed value
 </pre>
 
 The <a>mask border image</a> is drawn inside an area called the <dfn export>mask border image area</dfn>. This is an area whose boundaries by default correspond to the border box, see 'mask-border-outset'.
@@ -1071,7 +1071,7 @@ Inherited: no
 Percentages: n/a
 Computed value: all <<length>>s made absolute, otherwise as specified
 Media: visual
-Animation type: discrete
+Animation type: by computed value
 </pre>
 
 The values specify the amount by which the <a>mask border image area</a> extends beyond the border box. If it has four values, they set the outsets on the top, right, bottom and left sides in that order. If the left is missing, it is the same as the right; if the bottom is missing, it is the same as the top; if the right is missing, it is the same as the top.