Fix opacity of grayscale images

spec/pluto/format/png_spec.cr

@@ -8,7 +8,7 @@ describe Pluto::Format::PNG do
         io = IO::Memory.new
         image.to_png(io)
 
-        digest(io.to_s).should eq "944007df754a5737485981612977ce3d365ccb54"
+        digest(io.to_s).should eq "5fc8ce7e9d94d4b3ceb5d2216cf66e90e03c66e0"
       end
     end
 

spec/pluto/format/webp_spec.cr

@@ -8,7 +8,7 @@ describe Pluto::Format::WebP do
         io = IO::Memory.new
         image.to_webp(io)
 
-        digest(io.to_s).should eq "92678d201760ee50929ea344a81c76d5f61e70e6"
+        digest(io.to_s).should eq "4a23dfdec161349c163c0a17a6af4cf94e1807d7"
       end
     end
 

src/pluto/grayscale_image.cr

@@ -2,6 +2,7 @@ require "./image"
 
 class Pluto::GrayscaleImage < Pluto::Image
   property gray : Array(UInt8)
+  property alpha : Array(UInt8)
   property width : Int32
   property height : Int32
 
@@ -16,12 +17,13 @@ class Pluto::GrayscaleImage < Pluto::Image
     RGBAImage.new(red, green, blue, alpha, width, height).to_gray
   end
 
-  def initialize(@gray, @width, @height)
+  def initialize(@gray, @alpha, @width, @height)
   end
 
   def clone : GrayscaleImage
     self.class.new(
       @gray.clone,
+      @alpha.clone,
       @width,
       @height
     )
@@ -39,10 +41,6 @@ class Pluto::GrayscaleImage < Pluto::Image
     @gray
   end
 
-  def alpha : Array(UInt8)
-    @gray
-  end
-
   def each_channel(& : (Array(UInt8), ChannelType) -> Nil) : Nil
     yield @gray, ChannelType::Gray
     nil

src/pluto/rgba_image.cr

@@ -62,6 +62,7 @@ class Pluto::RGBAImage < Pluto::Image
           (red_pixel * red_multiplier + @green[index] * green_multiplier + @blue[index] * blue_multiplier).to_u8
         )
       end,
+      alpha,
       width,
       height
     )