Follow changes in WD-motion-1-20181218

See also: https://www.w3.org/TR/2018/WD-motion-1-20181218/

after/syntax/css/motion-1.vim

@@ -1,2 +1,2 @@
-syn match cssFontProp contained "\<offset\(-\(path\|distance\|anchor\|rotate\)\)\=\>"
+syn match cssFontProp contained "\<offset\(-\(path\|distance\|position\|anchor\|rotate\)\)\=\>"
 syn region cssFunction contained matchgroup=cssFunctionName start="\<\(ray\|path\)\s*(" end=")" oneline keepend

test/test.css

@@ -565,6 +565,7 @@ mask {
 .motion-1 {
   offset-path: auto;
   offset-distance: auto;
+  offset-position: auto;
   offset-anchor: auto;
   offset-rotate: auto;
   offset: auto;