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/logical/media_file.rb

@@ -202,5 +202,18 @@ class MediaFile
     }.stringify_keys
   end
 
+  # Scale `width` and `height` to fit within `max_width` and `max_height`.
+  def self.scale_dimensions(width, height, max_width, max_height)
+    max_width ||= Float::INFINITY
+    max_height ||= Float::INFINITY
+
+    if width <= max_width && height <= max_height
+      [width, height]
+    else
+      scale = [max_width.to_f / width.to_f, max_height.to_f / height.to_f].min
+      [(width * scale).round.to_i, (height * scale).round.to_i]
+    end
+  end
+
   memoize :file_ext, :file_size, :md5, :metadata
 end

app/logical/media_file/image.rb

@@ -72,8 +72,9 @@ class MediaFile::Image < MediaFile
     MediaFile::Image.new(output_file)
   end
 
-  def preview(max_width, max_height)
-    resize(max_width, max_height, size: :down)
+  def preview(max_width, max_height, **options)
+    w, h = MediaFile.scale_dimensions(width, height, max_width, max_height)
+    resize(w, h, size: :force, **options)
   end
 
   def crop(max_width, max_height)