posts: fix incorrect exif rotation for PNGs.

Fix a bug where where PNG images could be incorrectly detected as
exif-rotated. This would happen when a PNG contained the
IFD0:Orientation flag. It's technically possible for a PNG to contain
this flag, but it's ignored by libvips and by browsers.

post #3762340 (nsfw) is an example of a PNG like this.

The fix is to use `autorot` to let libvips apply the rotation instead of
trying to interpret the exif data ourselves. Note that libvips-8.9 has a
bug where it doesn't strip the orientation flag after applying
`autorot`, which leads to the image being incorrectly rotated a second
time when generating the thumbnail. Use libvips-8.11 instead.

test/unit/media_file_test.rb

@@ -359,4 +359,16 @@ class MediaFileTest < ActiveSupport::TestCase
       assert_equal([33, 50], @file.preview(50, 50).dimensions)
     end
   end
+
+  context "a PNG with an exif orientation flag" do
+    should "not generate rotated dimensions" do
+      @file = MediaFile.open("test/files/test-rotation-90cw.png")
+      assert_equal([128, 96], @file.dimensions)
+    end
+
+    should "not generate a rotated thumbnail" do
+      @file = MediaFile.open("test/files/test-rotation-90cw.png")
+      assert_equal([64, 48], @file.preview(64, 64).dimensions)
+    end
+  end
 end

test/unit/post_test.rb

@@ -1336,6 +1336,15 @@ class PostTest < ActiveSupport::TestCase
         end
       end
 
+      context "a PNG with the exif orientation flag" do
+        should "not add the exif_rotation tag" do
+          @media_asset = MediaAsset.create!(file: "test/files/test-rotation-90cw.png")
+          @post.update!(md5: @media_asset.md5)
+          @post.reload.update!(tag_string: "tagme")
+          assert_equal("tagme", @post.tag_string)
+        end
+      end
+
       context "a non-repeating GIF missing the non-repeating_animation tag" do
         should "automatically add the non-repeating_animation tag" do
           @media_asset = MediaAsset.create!(file: "test/files/test-animated-86x52-loop-1.gif")