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/components/post_preview_component.rb

@@ -42,13 +42,16 @@ class PostPreviewComponent < ApplicationComponent
     end
   end
 
-  def preview_dimensions
-    downscale_ratio = Danbooru.config.small_image_width.to_f / [post.image_width, post.image_height].max
+  def preview_width
+    variant.width
+  end
 
-    {
-      width: [(downscale_ratio * post.image_width).floor, post.image_width].min,
-      height: [(downscale_ratio * post.image_height).floor, post.image_height].min,
-    }
+  def preview_height
+    variant.height
+  end
+
+  def variant
+    @variant ||= media_asset.variant(:preview)
   end
 
   def tooltip

app/components/post_preview_component/post_preview_component.html.erb

@@ -16,7 +16,7 @@
     <%= tag.picture do -%>
       <%= tag.source media: "(max-width: 660px)", srcset: cropped_url -%>
       <%= tag.source media: "(min-width: 660px)", srcset: post.preview_file_url -%>
-      <%= tag.img class: "has-cropped-#{post.has_cropped?}", src: post.preview_file_url, style: "min-width: #{preview_dimensions[:width]}px; min-height: #{preview_dimensions[:height]}px;", title: tooltip, alt: "post ##{post.id}", crossorigin: "anonymous" -%>
+      <%= tag.img class: "has-cropped-#{post.has_cropped?}", src: post.preview_file_url, style: "min-width: #{preview_width}px; min-height: #{preview_height}px;", title: tooltip, alt: "post ##{post.id}", crossorigin: "anonymous" -%>
     <% end -%>
   <% end -%>
   <% if pool -%>