Initial patch to unify our render towards cairo

This branch allows a pure cairo based render for both PNG and SVG.

Cairo backend is mostly the same but with improved support for
configurable scale, orientation and inverted color drawing.

API is not yet final.

gerber/render/render.py

@@ -62,6 +62,7 @@ class GerberContext(object):
         self._drill_color = (0.25, 0.25, 0.25)
         self._background_color = (0.0, 0.0, 0.0)
         self._alpha = 1.0
+        self._invert = False
 
     @property
     def units(self):
@@ -122,6 +123,14 @@ class GerberContext(object):
             raise ValueError('Alpha must be between 0.0 and 1.0')
         self._alpha = alpha
 
+    @property
+    def invert(self):
+        return self._invert
+
+    @invert.setter
+    def invert(self, invert):
+        self._invert = invert
+
     def render(self, primitive):
         color = (self.color if primitive.level_polarity == 'dark'
                  else self.background_color)