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/intro/02_options.rb

@@ -4,11 +4,11 @@ Squib::Deck.new cards: 1 do
   background color: '#252322'
   rect fill_color: '#0B3736',
        x: 38, y: 38, width: 750, height: 1050, radius: 38
-  text str: 'Robot Golem', font: 'True Crimes, Sans 72',
+  text str: 'Robot Golem', font: 'True Crimes, Sans 24',
        align: :center, x: 75, width: :deck, color: '#DFDFE1', y: 90
   svg file: 'auto-repair.svg', x: 75, y: 75, width: 100, height: :scale
   svg file: 'robot-golem.svg', x: 75, y: 300, width: 675, height: :scale
-  text str: 'Draw two cards', font: 'Serif 36',
+  text str: 'Draw two cards', font: 'Serif 12',
        align: :center, width: :deck, color: '#DFDFE1', y: 1000
   save_png prefix: 'part2_'
 end

samples/intro/part3_layout.yml

@@ -6,7 +6,7 @@ backdrop:
     width: 750
     height: 1050
 title:
-    font: 'True Crimes, Sans 72'
+    font: 'True Crimes, Sans 24'
     align: center
     color: '#DFDFE1'
     x: 75
@@ -27,8 +27,8 @@ art:
     width: 675
     height: 675
 power:
-    font: 'Serif 36'
+    font: 'Serif 12'
     align: center
     width: 825
     color: '#DFDFE1'
-    y: 1000
+    y: 1000