Themeroller: Fix alpha treatment in icons

Since jQuery UI 1.13.0, partially transparent pixels in the icons file started
being too dark. Set gamma to 0.5 for the A channel to get closer to the 1.12.1
output.

A test comparing similarity of the generated image to the jQuery 1.12.1 has been
added as well.

Fixes jquery/jquery-ui#2339

Author: mgol

lib/themeroller-image.js

@@ -130,8 +130,11 @@ generateIcon = function( params, callback ) {
 
 	// https://usage.imagemagick.org/masking/#shapes
 	// See https://github.com/jquery/download.jqueryui.com/issues/132 for why
-	// `-set colorspace RGB` is needed (twice) in IM >6.7.9. Full command:
-	// $ magick <icons_mask_filename> -set colorspace RGB -background <color> -alpha shape -set colorspace sRGB output.png
+	// * `-set colorspace RGB` is needed (twice) in IM >6.7.9.
+	// * `-channel A -gamma 0.5` is needed so that partially transparent
+	// pixels are not too dark.
+	// Full command:
+	// $ magick <icons_mask_filename> -set colorspace RGB -background <color> -alpha shape -channel A -gamma 0.5 -set colorspace sRGB output.png
 
 	imageQueue.push( function( innerCallback ) {
 		try {
@@ -140,6 +143,7 @@ generateIcon = function( params, callback ) {
 				"-set", "colorspace", "RGB",
 				"-background", color,
 				"-alpha", "shape",
+				"-channel", "A", "-gamma", "0.45",
 				"-set", "colorspace", "sRGB",
 				"png:-"
 			], innerCallback );