Upgrade to latest version of ruby-vips

ruby-vips 2.0.11 adds Vips::Image#add_alpha and Vips::Image#has_alpha?,
which call the corresponding C functions directly, so we can now remove
our own implementations.

image_processing.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "mini_magick", "~> 4.0"
-  spec.add_dependency "ruby-vips", ">= 2.0.10", "< 3"
+  spec.add_dependency "ruby-vips", ">= 2.0.11", "< 3"
 
   spec.add_development_dependency "rake"
   spec.add_development_dependency "minitest", "~> 5.8"

lib/image_processing/vips.rb

@@ -45,7 +45,7 @@ def resize_and_pad(image, width, height, gravity: "centre", extend: nil, backgro
         embed_options.reject! { |name, value| value.nil? }
 
         image = thumbnail(image, width, height, **options)
-        image = add_alpha(image) if alpha && !has_alpha?(image)
+        image = image.add_alpha if alpha && !image.has_alpha?
         image.gravity(gravity, width, height, **embed_options)
       end
 
@@ -78,19 +78,6 @@ def thumbnail(image, width, height, sharpen: SHARPEN_MASK, **options)
         image
       end
 
-      # Port of libvips' vips_addalpha().
-      def add_alpha(image)
-        max_alpha = (image.interpretation == :grey16 || image.interpretation == :rgb16) ? 65535 : 255
-        image.bandjoin(max_alpha)
-      end
-
-      # Port of libvips' vips_hasalpha().
-      def has_alpha?(image)
-        image.bands == 2 ||
-        (image.bands == 4 && image.interpretation != :cmyk) ||
-        image.bands > 4
-      end
-
       def default_dimensions(width, height)
         raise Error, "either width or height must be specified" unless width || height
 

test/vips_test.rb

@@ -242,11 +242,14 @@
     end
 
     it "produces correct image when shrinking" do
-      expected = fixture_image("pad.png")
-      assert_similar expected, @pipeline.convert("png").resize_and_pad!(400, 400, alpha: true)
-
-      png = @pipeline.bandjoin(255).convert!("png")
-      assert_similar expected, @pipeline.source(png).resize_and_pad!(400, 400, alpha: true)
+      result = @pipeline.convert("png").resize_and_pad!(400, 400, alpha: true)
+      assert_similar fixture_image("pad.png"), result
+      assert_equal 4, Vips::Image.new_from_file(result.path).bands
+
+      transparent_png = @pipeline.add_alpha.convert!("png")
+      result = @pipeline.source(transparent_png).resize_and_pad!(400, 400, alpha: true)
+      assert_similar fixture_image("pad.png"), result
+      assert_equal 4, Vips::Image.new_from_file(result.path).bands
     end
 
     it "produces correct image when enlarging" do