posts: micro-optimize allocations during thumbnail generation.

Do a few micro-optimizations to reduce the number of memory allocations
during thumbnail generation.

This commit, combined with freezing string literals in a7dc05 and
67b961, reduces the number of allocations on the front page from 180,000
to 150,000, and the number of retained objects from 8,000 to 4,000.

app/models/media_asset.rb

@@ -4,6 +4,9 @@ class MediaAsset < ApplicationRecord
   class Error < StandardError; end
 
   VARIANTS = %i[preview crop 180x180 360x360 720x720 sample original]
+  ENABLE_SEO_POST_URLS = Danbooru.config.enable_seo_post_urls
+  LARGE_IMAGE_WIDTH = Danbooru.config.large_image_width
+  STORAGE_SERVICE = Danbooru.config.storage_manager
 
   has_one :media_metadata, dependent: :destroy
   has_one :pixiv_ugoira_frame_data, class_name: "PixivUgoiraFrameData", foreign_key: :md5, primary_key: :md5
@@ -88,7 +91,7 @@ class MediaAsset < ApplicationRecord
         "/images/download-preview.png"
       else
         slug = "__#{slug}__" if slug.present?
-        slug = nil if !Danbooru.config.enable_seo_post_urls
+        slug = nil if !ENABLE_SEO_POST_URLS
         "/#{variant}/#{md5[0..1]}/#{md5[2..3]}/#{slug}#{file_name}"
       end
     end
@@ -166,7 +169,7 @@ class MediaAsset < ApplicationRecord
       when :"720x720"
         true
       when :sample
-        media_asset.is_ugoira? || (media_asset.is_static_image? && media_asset.image_width > Danbooru.config.large_image_width)
+        media_asset.is_ugoira? || (media_asset.is_static_image? && media_asset.image_width > LARGE_IMAGE_WIDTH)
       when :original
         true
       end
@@ -271,7 +274,7 @@ class MediaAsset < ApplicationRecord
     end
 
     def storage_service
-      Danbooru.config.storage_manager
+      STORAGE_SERVICE
     end
 
     def backup_storage_service