Fallback to canvas measurement when font glyph not available

If characters are inserted for which the configured font has no glyph,
the code assigns a default width of 0 to the character. This causes the
line wrapping and cursor positioning logic to be incorrect.

Instead of assigning a default width of 0, fallback to a canvas-based
measurement to determine the width the browser will use when rendering the
character.

Fixes #11.

src/core/TextFontMetrics.js

@@ -49,7 +49,8 @@ function calcCharAdvanceOpenType(char, fontSize, font, unitsPerEm) {
   let glyph = font.charToGlyph(char)
   return glyph.unicode ?
     glyph.advanceWidth * calcFontScale(fontSize, unitsPerEm) :
-    0
+    // font doesn't contain a glyph for this char, fallback to canvas measurement
+    calcTextAdvanceCanvas(char, fontSize, font)
 }
 
 let canvas = _.memoize(function() {