Correctly handle "use"ing symbols

The svg spec says to treat "use"d symbols as svg elements, and to
transfer the width and height from the use element to the cloned svg
element.  This fixes a rendering problem reported in issue #157.

lib/prawn/svg/elements/use.rb

@@ -1,15 +1,12 @@
 class Prawn::SVG::Elements::Use < Prawn::SVG::Elements::Base
-  attr_reader :referenced_element_class
-  attr_reader :referenced_element_source
+  attr_reader :referenced_element_class, :referenced_element_source
 
   def parse
     href = href_attribute
-    if href.nil?
-      raise SkipElementError, "use tag must have an href or xlink:href"
-    end
+    raise SkipElementError, 'use tag must have an href or xlink:href' if href.nil?
 
     if href[0..0] != '#'
-      raise SkipElementError, "use tag has an href that is not a reference to an id; this is not supported"
+      raise SkipElementError, 'use tag has an href that is not a reference to an id; this is not supported'
     end
 
     id = href[1..-1]
@@ -29,14 +26,18 @@ def parse
       end
     end
 
-    if referenced_element_class.nil?
-      raise SkipElementError, "no tag with ID '#{id}' was found, referenced by use tag"
+    raise SkipElementError, "no tag with ID '#{id}' was found, referenced by use tag" if referenced_element_class.nil?
+
+    if referenced_element_source.name == 'symbol'
+      @referenced_element_class = Prawn::SVG::Elements::Viewport
     end
 
     state.inside_use = true
 
     @x = attributes['x']
     @y = attributes['y']
+    @width = attributes['width']
+    @height = attributes['height']
   end
 
   def container?
@@ -45,16 +46,23 @@ def container?
 
   def apply
     if @x || @y
-      add_call_and_enter "translate", x_pixels(@x || 0), -y_pixels(@y || 0)
+      add_call_and_enter 'translate', x_pixels(@x || 0), -y_pixels(@y || 0)
     end
   end
 
   def process_child_elements
-    add_call "save"
+    add_call 'save'
+
+    source = referenced_element_source.dup
+
+    if referenced_element_class == Prawn::SVG::Elements::Viewport
+      source.attributes['width'] = @width || '100%'
+      source.attributes['height'] = @height || '100%'
+    end
 
-    child = referenced_element_class.new(document, referenced_element_source, calls, state.dup)
+    child = referenced_element_class.new(document, source, calls, state.dup)
     child.process
 
-    add_call "restore"
+    add_call 'restore'
   end
 end