Add image_size gem to test screenshots

ruby-vnc.gemspec

@@ -36,4 +36,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake', '~> 12.3'
   s.add_development_dependency 'rspec', '~> 3.7'
   s.add_development_dependency 'simplecov', '~> 0.16'
+  s.add_development_dependency 'image_size', '~> 2.0'
 end

spec/real_net_vnc_spec.rb

@@ -45,6 +45,13 @@
   end
 
   context 'screenshotting' do
+    def verify_screenshot(input)
+      image_size = ImageSize.path(input)
+      expect(image_size.format).to eq :png
+      expect(image_size.width).to eq 1366
+      expect(image_size.height).to eq 768
+    end
+
     it 'should allow you to take a screenshot with a path' do
       Tempfile.open('ruby-vnc-spec') do |screenshotfile|
         Net::VNC.open(NO_AUTH_SERVER_DISPLAY) do |vnc|

spec/spec_helper.rb

@@ -17,6 +17,9 @@
 require 'simplecov'
 SimpleCov.start
 
+require 'tempfile'
+require 'image_size'
+
 RSpec.configure do |config|
   # rspec-expectations config goes here. You can use an alternate
   # assertion/expectation library such as wrong or the stdlib/minitest