Fix #4612: Input profile error with greyscale jpg images.

Fix a bug where generating thumbnails failed for certain images when
using libvips 8.10. Specifically, it failed for single-channel greyscale
images and four-channel CMYK images without an embedded color profile.
In these cases we specified an sRGB fallback profile, but under libvips
8.10 this failed because the sRGB profile was incompatible with
single-channel and four-channel images. Before libvips 8.10 this worked,
but as of 8.10 it's a hard error.

The way libvips handles fallback color profiles differs across versions,
so we have to use different arguments for different versions. In 8.7,
vips doesn't have builtin color profiles, so we have to specify our own
manually. In 8.9, it has builtin profiles, so we can omit the import
profile, but we're still required to set the export profile to sRGB,
otherwise it will leave CMYK images as CMYK when generating thumbnails.
In 8.10, we have to _not_ to set the import or export profile to sRGB,
otherwise it will fail with an incompatible profile error when it tries
to convert CMYK images to RGB.

The builtin sRGB profile used by libvips[1] is different than the one we
used previously[2]. The builtin one comes from LCMS[3], whereas ours
came from ArgyllCMS.[4] Not all sRGB profiles are created the same[5],
so this may result in some imperceptible differences in thumbnail
output. The ArgyllCMS profile was used before because it seemed to be
the best one[6], but realistically it probably doesn't matter.

1: https://github.com/libvips/libvips/blob/v8.10.6/libvips/colour/profiles/sRGB.icm
2: 906eec190d/config/sRGB.icm
3: https://www.littlecms.com/
4: https://www.argyllcms.com/
5: https://ninedegreesbelow.com/photography/srgb-profile-comparison.html
6: https://ninedegreesbelow.com/photography/srgb-profile-comparison.html#addendum

test/unit/media_file_test.rb

@@ -179,4 +179,34 @@ class MediaFileTest < ActiveSupport::TestCase
       assert_equal(false, MediaFile.open("test/files/test-300x300.mp4").has_audio?)
     end
   end
+
+  context "a greyscale image without an embedded color profile" do
+    should "successfully generate a thumbnail" do
+      @image = MediaFile.open("test/files/test-grey-no-profile.jpg")
+      @preview = @image.preview(150, 150)
+
+      assert_equal(1, @image.channels)
+      assert_equal(:"b-w", @image.colorspace)
+      assert_equal([535, 290], @image.dimensions)
+
+      assert_equal(3, @preview.channels)
+      assert_equal(:srgb, @preview.colorspace)
+      assert_equal([150, 81], @preview.dimensions)
+    end
+  end
+
+  context "a CMYK image without an embedded color profile" do
+    should "successfully generate a thumbnail" do
+      @image = MediaFile.open("test/files/test-cmyk-no-profile.jpg")
+      @preview = @image.preview(150, 150)
+
+      assert_equal(4, @image.channels)
+      assert_equal(:cmyk, @image.colorspace)
+      assert_equal([197, 256], @image.dimensions)
+
+      assert_equal(3, @preview.channels)
+      assert_equal(:srgb, @preview.colorspace)
+      assert_equal([115, 150], @preview.dimensions)
+    end
+  end
 end