Respect DPI settings in text, add pt unit

This has the compatibility impact that all font sizes should be divided by 3 to maintain their same sizes.

Based on PR #225. Thanks @felixleong!!

Conflicts:
	bin/squib
	lib/squib/graphics/text.rb

samples/config/config_text_markup.rb

@@ -3,18 +3,18 @@ require 'squib'
 Squib::Deck.new(config: 'config_text_markup.yml') do
   background color: :white
   text str: %{"'Yaml ain't markup', he says"},
-       x: 10, y: 10, width: 300, height: 200, font: 'Serif 20',
+       x: 10, y: 10, width: 300, height: 200, font: 'Serif 7',
        markup: true, hint: :cyan
 
   text str: 'Notice also the antialiasing method.',
-       x: 320, y: 10, width: 300, height: 200, font: 'Arial Bold 20'
+       x: 320, y: 10, width: 300, height: 200, font: 'Arial Bold 7'
 
   save_png prefix: 'config_text_'
 end
 
 Squib::Deck.new(config: 'config_disable_quotes.yml') do
   text str: %{This has typographic sugar --- and ``explicit'' quotes --- but the quotes are "dumb"},
-       x: 10, y: 10, width: 300, height: 200, font: 'Serif 20',
+       x: 10, y: 10, width: 300, height: 200, font: 'Serif 7',
        markup: true, hint: :cyan
   save_png prefix: 'config_disable_text_'
 end

samples/config/custom_config.rb

@@ -6,7 +6,7 @@ Squib::Deck.new(config: 'custom-config.yml') do
 
   # Hints can be turned on in the config file
   text str: 'The Title', x: 0, y: 78, width: 825,
-       font: 'Arial 72', align: :center
+       font: 'Arial 24', align: :center
 
   # Progress bars are shown for these commands
   # And images are taken from img_dir, not the cwd.