Document image drawing in Kiva

[jwiggins]

Closes #616

[aaronayres35]

LGTM

Unrelated to this PR, but while reviewing I went through the kiva examples looking for an image one. running the image_explorer.py example I saw the following error:

$ python image_explorer.py 
2021-03-04 12:20:39.724 Python[18846:396139] ApplePersistenceIgnoreState: Existing state will not be touched. New state will be written to (null)
Traceback (most recent call last):
  File "/Users/aayres/Desktop/enable/enable/qt4/base_window.py", line 243, in paintEvent
    self.handler.paintEvent(event)
  File "/Users/aayres/Desktop/enable/enable/qt4/base_window.py", line 78, in paintEvent
    self._enable_window._paint(event)
  File "/Users/aayres/Desktop/enable/enable/abstract_window.py", line 506, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File "/Users/aayres/Desktop/enable/enable/component.py", line 432, in draw
    self._draw(gc, view_bounds, mode)
  File "/Users/aayres/Desktop/enable/enable/component.py", line 813, in _draw
    self._dispatch_draw(layer, gc, view_bounds, mode)
  File "/Users/aayres/Desktop/enable/enable/component.py", line 831, in _dispatch_draw
    handler(gc, view_bounds, mode)
  File "image_explorer.py", line 40, in _draw_mainlayer
    gc.draw_image(self.image, (0, 0, gc.width(), gc.height()))
  File "/Users/aayres/Desktop/enable/kiva/agg/agg.py", line 1379, in draw_image
    return _agg.GraphicsContextArray_draw_image(self,img,rect,force_copy)
TypeError: in method 'GraphicsContextArray_draw_image', argument 2 of type 'kiva::graphics_context_base *'
Abort trap: 6

This probably warrants an issue?

[jwiggins]

Thanks for the feedback.

[jwiggins]

#681 added to track the image_explorer.py issue.