fixes for builder get() procs and cairo

examples/darea_test.nim

@@ -0,0 +1,25 @@
+import gintro/cairo
+import drawingarea
+from math import PI
+
+proc extents(): (float, float, float, float) =
+  (0.0, 0.0, 100.0, 100.0) # ugly float literals
+
+# draw to cairo context
+proc draw(cr: cairo.Context) =
+  cr.setSource(1, 1, 1) # set background color and paint
+  cr.paint
+  cr.setSource(0, 0, 0) # forground color
+  cr.arc(20, 30, 10, 0, 5) # nearly a circle
+  cr.newSubPath # do not join the two arcs
+  cr.arc(70, 60, 20, 0, math.PI)
+  cr.stroke # finally do it
+
+proc main =
+  var data: PDA_Data
+  data.draw = draw
+  data.extents = extents
+  data.windowSize = (800, 600)
+  newDisplay(data)
+
+main()