FEATURE: introduce lossy color optimization on resized pngs

This feature ensures optimized images run via pngquant, this results extreme amounts of savings for resized images. Effectively the only impact is that the color palette on small resized images is reduced to 256.

To ensure safety we only apply this optimisation to images smaller than 500k.

This commit also makes a bunch of image specs less fragile.

spec/lib/upload_creator_spec.rb

@@ -98,6 +98,27 @@ RSpec.describe UploadCreator do
       end
     end
 
+    describe 'pngquant' do
+      let(:filename) { "pngquant.png" }
+      let(:file) { file_from_fixtures(filename) }
+
+      it 'should apply pngquant to optimized images' do
+        upload = UploadCreator.new(file, filename,
+          pasted: true,
+          force_optimize: true
+        ).create_for(user.id)
+
+        # no optimisation possible without losing details
+        expect(upload.filesize).to eq(9558)
+
+        thumbnail_size = upload.get_optimized_image(upload.width, upload.height, {}).filesize
+
+        # pngquant will lose some colors causing some extra size reduction
+        expect(thumbnail_size).to be < 7500
+      end
+
+    end
+
     describe 'converting to jpeg' do
       let(:filename) { "should_be_jpeg.png" }
       let(:file) { file_from_fixtures(filename) }