Place embedded icons based on byte offset

Fixes #57

lib/squib/graphics/text.rb

@@ -119,7 +119,8 @@ module Squib
       while (key = next_embed(embed.rules.keys, clean_str)) != nil
         rule          = embed.rules[key]
         spacing       = rule[:width] * Pango::SCALE
-        index         = clean_str.index(key)
+        index         = clean_str.index(key) 
+        index         = clean_str[0..index].bytesize #convert to byte index (bug #57)
         str = str.sub(key, "<span size=\"#{ZERO_WIDTH_CHAR_SIZE}\">a<span letter_spacing=\"#{spacing.to_i}\">a</span>a</span>")
         layout.markup = str
         clean_str     = layout.text

samples/embed_text.rb

@@ -66,8 +66,8 @@ Squib::Deck.new do
     embed.png key: ':purse:',  width: 28, height: 28, file: 'shiny-purse.png'
   end
 
-  embed_text = 'Markup and typography replacements with ":tool:" icons <i>won\'t</i> fail'
-  text(str: embed_text, font: 'Serif 21', markup: true,
+  embed_text = 'Markup --- and typography replacements --- with ":tool:" icons <i>won\'t</i> fail'
+  text(str: embed_text, font: 'Serif 18', markup: true,
        x: 600, y: 320, width: 180, height: 300,
        align: :center, hint: :magenta) do |embed|
     embed.svg key: ':tool:',   width: 28, height: 28, file: 'spanner.svg'

spec/data/samples/embed_text.rb.txt

@@ -185,7 +185,7 @@ cairo: translate([600, 320])
 cairo: rotate([0])
 cairo: move_to([0, 0])
 pango: font_description=([MockDouble])
-pango: text=(["Markup and typography replacements with \":tool:\" icons <i>won't</i> fail"])
+pango: text=(["Markup --- and typography replacements --- with \":tool:\" icons <i>won't</i> fail"])
 pango: markup=(["foo"])
 pango: width=([184320])
 pango: height=([307200])