Feat/font autoscale (#288)

* PoC font autosize feature

* Take enumerables or 'auto' keyword for autoscaling

* Allo enumerables of font_size per card, where the largest font size that does not ellipsize is chosen

* Use ellipsize: :autoscale keyword to determine font autosizing

* Use binary search to find the correct font size

* Consider new ellipsize option in testcase

* Document ellipsize: :autoscale option

* Add ellipsize: :autoscale sample to text sample

* Fix double multiplication with Pango::SCALE

* Scale embedded images with text when autoscaling

* Remove stray semicolon

* Return dummy value for FontDescription.size in mock_cairo

* Update regression test data for autoscale feature.

* Add ellipsize: :autoscale example to existing font_autoscale sample

* Add missing :autoscale to list of options in documentation

samples/autoscale_font/_autoscale_font.rb

@@ -16,14 +16,20 @@ def autoscale(str_array)
   end
 end
 
-Squib::Deck.new(width: 300, height: 100, cards: 3) do
+Squib::Deck.new(width: 300, height: 200, cards: 3) do
   background color: :white
   rect
   title = ['Short & Big',
            'Medium Length & Size',
            'Super duper long string here, therefore a smaller font.']
+  
+  # Scale text based on the string length
   text str: title, font: 'Arial', font_size: autoscale(title),
        x: 10, y:10, align: :center, width: 280, ellipsize: false, hint: :red
+  
+  # Automatically scale the text down from the specified font_size to the largest size that fits
+  text str: title, font: 'Arial', font_size: 10,
+       x: 10, y:100, align: :center, width: 280, height: 50, ellipsize: :autoscale, hint: :red
 
   save_sheet columns: 3
 end