add advanceWidth to glyphRun

pitfall/fontkit/glyphrun.rkt

@@ -2,9 +2,18 @@
 (require "bbox.rkt" (prefix-in Script- "script.rkt"))
 (provide (all-defined-out))
 
+#|
+approximates
+https://github.com/mbutterick/fontkit/blob/master/src/layout/GlyphRun.js
+|#
+
 ;; Represents a run of Glyph and GlyphPosition objects.
 ;; Returned by the font layout method.
 (define-subclass object% (GlyphRun
                           glyphs ; An array of Glyph objects in the run
                           positions) ; An array of GlyphPosition objects for each glyph in the run
-                          )
+
+
+  (define/public (advanceWidth)
+    (for/sum ([pos (in-list positions)])
+             (· pos xAdvance))))