creating url thumbnail generator

Signed-off-by: man13 <man13@inf.ufpr.br>

lib/thumbnail/strategies/url_thumbnail_generator.rb

+require 'screencap'
+
+module Thumbnail
+  module Strategies
+    class UrlThumbnailGenerator < ::Thumbnail::GeneratableStrategy
+
+      def generate(screen)
+        screen.fetch(
+            width: 1024,
+            height: 768
+        )
+      end
+
+      def can_generate?(screen)
+        ::Screencap::Fetcher == screen.class
+      end
+
+    end
+  end
+end
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