Ginga: Switch from aggdraw to pycairo

[pllim]

@ejeschke said pycairo will provide the most capability. Perhaps he can elaborate.

[ejeschke]

Thinking about this a little more, I think I withdraw the comment--aggdraw should be as capable as pycairo, and probably faster as well. However, pycairo, pillow and opencv are all installable even in default conda environments, whereas aggdraw you have to install from git, presently. pycairo does support all (I think) attributes of ginga canvas objects line properties, such as dashed lines, various line thicknesses, full alpha fills with transparency, etc.

aggdraw does also, IIRC, but since pycairo is in the conda environment and used by so many other projects, it's probably the better choice. As Ginga will check to see what is installed and pick the preferred renderer (cairo), I don't think you need to do anything special on the astrowidgets side unless you are specifically requesting a particular renderer.

[pllim]

@ejeschke , should we update https://astrowidgets.readthedocs.io/en/latest/install.html to omit aggdraw and replace the dependency list with cairo then?