post tooltips: remove gap beneath thumbnail on post #xxx tooltips.

Set the thumbnail height to auto instead of 154px so that there's not a
big empty gap beneath thumbnails that aren't 150px high.

This requires setting dimensions on the <img> tag itself so that the
image height is known before loading, otherwise the tooltip will be
mispositioned after the image loads in.

We set the min-width and min-height instead of the <img> width and
height properties because our calculated dimensions are sometimes
off-by-one compared to the actual dimensions. I'm not sure how libvips
calculates the thumbnail dimensions, but this avoids forcing a slightly
wrong aspect ratio, which produces ugly resizing artifacts.

app/presenters/post_presenter.rb

@@ -63,6 +63,10 @@ class PostPresenter < Presenter
     locals[:width] = post.image_width
     locals[:height] = post.image_height
 
+    downscale_ratio = Danbooru.config.small_image_width.to_f / [post.image_width, post.image_height].max
+    locals[:preview_width] = [(downscale_ratio * post.image_width).floor, post.image_width].min
+    locals[:preview_height] = [(downscale_ratio * post.image_height).floor, post.image_height].min
+
     if options[:similarity]
       locals[:similarity] = options[:similarity].round
     else