Use chunky_png/oily_png :fast_rgb option when saving

According to the chunky_png wiki:

> ChunkyPNG tries to detect the optimal encoding parameters when saving
> an image. The detection costs some time, and it may choose a slow
> encoding method. To override the detection mechanism, and always save
> as RGB or RGBA image, use the @:fast_rgba@ or @:fast_rgb@ modifier
> when calling save

https://github.com/wvanbergen/chunky_png/wiki#encoding-options

Since we know that we are always only dealing with RGB images here and
speed is more important to us than space for this use-case, we can get a
nice little win here.

This brought the runtime on the Brigade diffux test suite down from
~170s to ~132s.

lib/diffux_ci_runner.rb

@@ -143,11 +143,11 @@ def init_driver
           # image to disk. This will allow it to be reviewed by someone.
           diff_path = DiffuxCIUtils.path_to(
             description, viewport['name'], 'diff.png')
-          comparison[:diff_image].save(diff_path)
+          comparison[:diff_image].save(diff_path, :fast_rgb)
 
           candidate_path = DiffuxCIUtils.path_to(
             description, viewport['name'], 'candidate.png')
-          screenshot.save(candidate_path)
+          screenshot.save(candidate_path, :fast_rgb)
 
           puts "#{comparison[:diff_in_percent].round(1)}% (#{candidate_path})"
         else
@@ -163,7 +163,7 @@ def init_driver
         unless File.directory?(dirname = File.dirname(baseline_path))
           FileUtils.mkdir_p(dirname)
         end
-        screenshot.save(baseline_path)
+        screenshot.save(baseline_path, :fast_rgb)
         puts "First snapshot created (#{baseline_path})"
       end
     end