uploads: move thumbnail generation code to MediaFile.

* Move image thumbnail generation code to MediaFile::Image.
* Move video thumbnail generation code to MediaFile::Video.
* Move ugoira->webm conversion code to MediaFile::Ugoira.

This separates thumbnail generation from the upload process so that it's
possible to generate thumbnails outside of uploads.

app/logical/media_file/image.rb

@@ -1,9 +1,46 @@
 class MediaFile::Image < MediaFile
-  def dimensions
-    image.size
+  # Taken from ArgyllCMS 2.0.0 (see also: https://ninedegreesbelow.com/photography/srgb-profile-comparison.html)
+  SRGB_PROFILE = "#{Rails.root}/config/sRGB.icm"
+
+  # http://jcupitt.github.io/libvips/API/current/VipsForeignSave.html#vips-jpegsave
+  JPEG_OPTIONS = { Q: 90, background: 255, strip: true, interlace: true, optimize_coding: true }
+
+  # http://jcupitt.github.io/libvips/API/current/libvips-resample.html#vips-thumbnail
+  if Vips.at_least_libvips?(8, 8)
+    THUMBNAIL_OPTIONS = { size: :down, linear: false, no_rotate: true, export_profile: SRGB_PROFILE, import_profile: SRGB_PROFILE }
+    CROP_OPTIONS = { crop: :attention, linear: false, no_rotate: true, export_profile: SRGB_PROFILE, import_profile: SRGB_PROFILE }
+  else
+    THUMBNAIL_OPTIONS = { size: :down, linear: false, auto_rotate: false, export_profile: SRGB_PROFILE, import_profile: SRGB_PROFILE }
+    CROP_OPTIONS = { crop: :attention, linear: false, auto_rotate: false, export_profile: SRGB_PROFILE, import_profile: SRGB_PROFILE }
   end
 
+  def dimensions
+    image.size
+  rescue Vips::Error
+    [0, 0]
+  end
+
+  # https://github.com/jcupitt/libvips/wiki/HOWTO----Image-shrinking
+  # http://jcupitt.github.io/libvips/API/current/Using-vipsthumbnail.md.html
+  def preview(width, height)
+    output_file = Tempfile.new(["image-preview", ".jpg"])
+    resized_image = image.thumbnail_image(width, height: height, **THUMBNAIL_OPTIONS)
+    resized_image.jpegsave(output_file.path, **JPEG_OPTIONS)
+
+    MediaFile::Image.new(output_file)
+  end
+
+  def crop(width, height)
+    output_file = Tempfile.new(["image-crop", ".jpg"])
+    resized_image = image.thumbnail_image(width, height: height, **CROP_OPTIONS)
+    resized_image.jpegsave(output_file.path, **JPEG_OPTIONS)
+
+    MediaFile::Image.new(output_file)
+  end
+
+  private
+
   def image
-    @image ||= Vips::Image.new_from_file(file.path)
+    @image ||= Vips::Image.new_from_file(file.path, fail: true)
   end
 end