Fix PDF scaling issue

Properly compute the size of the PDF based on DPI and PDF-specific resolution (i.e. 72dpi).

Closes #62

spec/data/samples/custom_config.rb.txt

@@ -35,6 +35,7 @@ cairo: scale([1.0, 1.0])
 cairo: render_rsvg_handle([RSVG::Handle, nil])
 cairo: restore([])
 surface: write_to_png(["_output/custom-config_00.png"])
+cairo: scale([0.24, 0.24])
 cairo: translate([75, 75])
 cairo: rectangle([0, 0, 825, 1125])
 cairo: clip([])

spec/data/samples/saves.rb.txt

@@ -494,6 +494,7 @@ cairo: move_to([0, 0])
 cairo: update_pango_layout([MockDouble])
 cairo: show_pango_layout([MockDouble])
 cairo: restore([])
+cairo: scale([0.24, 0.24])
 cairo: translate([75, 75])
 cairo: rectangle([37, 37, 751, 1051])
 cairo: clip([])
@@ -608,6 +609,7 @@ cairo: paint([])
 cairo: reset_clip([])
 cairo: translate([-2343, -1131])
 cairo: show_page([])
+cairo: scale([0.24, 0.24])
 cairo: translate([75, 75])
 cairo: rectangle([0, 0, 825, 1125])
 cairo: clip([])

spec/graphics/graphics_save_doc_spec.rb

@@ -28,6 +28,7 @@ describe Squib::Deck, '#save_pdf' do
       deck = Squib::Deck.new(cards: 9, width: 825, height: 1125)
       expect(deck).to receive(:dirify) { |arg| arg } #don't create the dir
       expect(Squib.logger).to receive(:debug).at_least(:once)
+      expect(cxt).to receive(:scale).with(0.24, 0.24)
 
       expect_card_place(75, 75)
       expect_card_place(831, 75)
@@ -50,6 +51,7 @@ describe Squib::Deck, '#save_pdf' do
       deck = Squib::Deck.new(cards: num_cards, width: 825, height: 1125)
       expect(Squib.logger).to receive(:debug).at_least(:once)
       expect(deck).to receive(:dirify) { |arg| arg }  #don't create the dir
+      expect(cxt).to receive(:scale).with(0.24, 0.24)
 
       expect_card_place(75, 75)
       expect_card_place(831, 75)