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/colors/_colors.rb

@@ -26,7 +26,7 @@ Squib::Deck.new(width: 3000, height: 1500) do
   x, y, w, h = 0, 0, 300, 50
   colors.each_with_index do |color, i|
     rect x: x, y: y, width: w, height: h, fill_color: color
-    text str: color.to_s, x: x + 5, y: y + 13, font: 'Sans Bold 16',
+    text str: color.to_s, x: x + 5, y: y + 13, font: 'Sans Bold 5',
          color: (Cairo::Color.parse(color).to_hsv.v > 0.9) ? '#000' : '#fff'
     y += h
     if y > @height

samples/colors/_gradients.rb

@@ -27,7 +27,7 @@ Squib::Deck.new do
        radius: 15, stroke_color: '#0000'
 
   # Alpha transparency can be used too
-  text str: 'Hello, world!', x: 75, y: 700, font: 'Sans Bold 72',
+  text str: 'Hello, world!', x: 75, y: 700, font: 'Sans Bold 24',
        color: '(0,0)(825,0) #000f@0.0 #0000@1.0'
 
   save_png prefix: 'gradient_'