Add rewriting of sourceMappingURL comment for css

[fcheung]

Previously only javascript files had their sourceMappingURL comments rewritten with the digested asset location. This PR adds the same treatment to sourceMappingURL comments in css files

[ghost]

Hi @fcheung, consider hooking this up with register_bundle_processor as otherwise the annotation will get clobbered by e.g. autoprefixer-rails.

I've temporarily worked around the issue while waiting for your PR to be merged by doing the following:

# config/initializers/source_maps.rb
class CustomSourcemappingUrlProcessor < Sprockets::Rails::SourcemappingUrlProcessor
  FIXED_REGEX = %r{(/(?:/|\*))# sourceMappingURL=(?:.*/)?(.*\.map)(?:;)(.*?)$}.freeze

  class << self
    def call(input)
      env     = input[:environment]
      context = env.context_class.new(input)
      data    = input[:data].gsub(FIXED_REGEX) do |_match|
        sourcemap_logical_path = combine_sourcemap_logical_path(sourcefile: input[:name], sourcemap: ::Regexp.last_match(2))

        begin
          resolved_sourcemap_comment(sourcemap_logical_path, context: context, prefix: ::Regexp.last_match(1), suffix: ::Regexp.last_match(3))
        rescue Sprockets::FileNotFound
          removed_sourcemap_comment(sourcemap_logical_path, filename: input[:filename], env: env)
        end
      end

      { data: data }
    end

    private

    def resolved_sourcemap_comment(sourcemap_logical_path, context:, prefix:, suffix:)
      "#{prefix}# sourceMappingURL=#{sourcemap_asset_path(sourcemap_logical_path, context: context)}#{suffix}\n#{prefix} !#{suffix}\n"
    end
  end
end

Rails.application.config.assets.configure do |env|
  # Remove broken processor
  env.unregister_postprocessor 'application/javascript', Sprockets::Rails::SourcemappingUrlProcessor

  # Replace with working processor, make sure it's placed after autoprefixer-rails' one
  # by placing it in a config.assets.configure block and registering it as a bundle processor
  env.register_bundle_processor 'text/css', CustomSourcemappingUrlProcessor
  env.register_bundle_processor 'application/javascript', CustomSourcemappingUrlProcessor
end