draw_image() should accept arrays, too

[jwiggins]

Currently, colorbars are drawn by initializing a separate backend_image.GraphicsContext with an array. This GraphicsContext is then drawn onto the target context. Other backends, even the GUI backends that use backend_image for its core implementation, do not accept arrays as arguments to its constructor. Instead, a GraphicsContext of the appropriate backend should be initialized as empty and then the colorbar data should be drawn onto it with draw_image().

This will help with the Mac Quartz backend as well as the noninteractive vector formats (PDF, PS, SVG).

https://svn.enthought.com/enthought/ticket/996

[jwiggins]

most backends (quartz, gl, pdf, cairo, qpainter) already support this

[jwiggins]

Closed by #590