Fix QPainter backend image drawing

[jwiggins]

The test kiva.tests.test_qpainter_drawing.TestQPainterDrawing.test_image is currently disabled because there appears to be some color confusion in the QPainter kiva backend. The test draws an image containing a red square and checks the pixels drawn to see if any are red. However no pixels are red because the QPainter code draws a blue square instead. When a numpy array is passed to draw_image, it is assumed to be in RGB order (QImage.Format_RGB888 or QImage.Format_RGB32) but ends up getting drawn as if it were in BGR order.

https://www.riverbankcomputing.com/static/Docs/PyQt5/api/qtgui/qimage.html#qimage-image-formats
https://www.riverbankcomputing.com/static/Docs/PyQt4/qimage.html#Format-enum