MediaFile: fix thumbnail dimension calculation.

Calculate the dimensions of thumbnails ourselves instead of letting
libvips calculate them for us. This way we know the exact size of
thumbnails, so we can set the right width and height for <img> tags. If
we let libvips calculate thumbnail sizes for us, then we can't predict
the exact size of thumbnails, because sometimes libvips rounds numbers
differently than us.

app/models/media_asset.rb

@@ -25,12 +25,14 @@ class MediaAsset < ApplicationRecord
   validates :md5, uniqueness: { conditions: -> { where(status: [:processing, :active]) } }
 
   class Variant
+    extend Memoist
+
     attr_reader :media_asset, :variant
     delegate :md5, :storage_service, :backup_storage_service, to: :media_asset
 
     def initialize(media_asset, variant)
       @media_asset = media_asset
-      @variant = variant
+      @variant = variant.to_sym
 
       raise ArgumentError, "asset doesn't have #{variant} variant" unless Variant.exists?(media_asset, variant)
     end
@@ -55,13 +57,13 @@ class MediaAsset < ApplicationRecord
     def convert_file(media_file)
       case variant
       in :preview
-        media_file.preview(Danbooru.config.small_image_width, Danbooru.config.small_image_width)
+        media_file.preview(width, height)
       in :crop
-        media_file.crop(Danbooru.config.small_image_width, Danbooru.config.small_image_width)
+        media_file.crop(width, height)
       in :sample if media_asset.is_ugoira?
         media_file.convert
-      in :sample if media_asset.is_static_image? && media_asset.image_width > Danbooru.config.large_image_width
-        media_file.preview(Danbooru.config.large_image_width, media_asset.image_height)
+      in :sample if media_asset.is_static_image?
+        media_file.preview(width, height)
       in :original
         media_file
       end
@@ -105,6 +107,36 @@ class MediaAsset < ApplicationRecord
       end
     end
 
+    def max_dimensions
+      case variant
+      when :preview
+        [150, 150]
+      when :crop
+        [150, 150]
+      when :sample
+        [850, nil]
+      when :original
+        [nil, nil]
+      end
+    end
+
+    def dimensions
+      case variant
+      when :crop
+        max_dimensions
+      else
+        MediaFile.scale_dimensions(media_asset.image_width, media_asset.image_height, max_dimensions[0], max_dimensions[1])
+      end
+    end
+
+    def width
+      dimensions[0]
+    end
+
+    def height
+      dimensions[1]
+    end
+
     def self.exists?(media_asset, variant)
       case variant
       when :preview
@@ -117,6 +149,8 @@ class MediaAsset < ApplicationRecord
         true
       end
     end
+
+    memoize :file_name, :file_ext, :max_dimensions, :dimensions
   end
 
   concerning :SearchMethods do