ipynb_static backend
This backend allows us to draw a single png image onto IPython notebook.
- It imports `display_png` from `IPython.display`
- Takes a screenshot, converts it to PNG and encodes with base64
- Displays it with `display_png` on `canvas.show()`
Add examples
Modified ModularVisual to check dimension of color, added reactive polygon test and added color to visuals and examples

Minor fix post rebase

Fixed spelling mistake

Added reactive ellipse tests
Implement (partly) proper way to inherit from Entity

That is, each Subclass does `__init__(what=None, I=3, need=3, **kwargs)`
and then `SuperClass.__init__(self, **kwargs)`

Also fixes to Text class and SceneCanvas can be other bgcolor than
black :)
Changed coordinates of test visuals
Added reactive regular polygon tests
Fixed  regular polygon tests
Address reviewer comments

I incorporated changes from #358, so that when that PR gets
rebased, it should result in minimal clashes.
flakify
Adds egl API

It is not 100% complete and the code to load the lib should
be generalized, but this should be a good starting point.
I had a working demo with this some time ago.
Make ipynb_vnc not tested
Merge pull request #381 from almarklein/lib-egl

Adds egl API
address comments
Merge pull request #378 from almarklein/entity-inheritance

Implement (partly) proper way to inherit from Entity
Replaced color and border_color attributes as Color objects, added Color.is_blank() method
ENH: Add entity print_tree function
ENH: Add transform option
STY: Remove unnecessary import
Merge pull request #384 from Eric89GXL/tree

ENH: Add entity print_tree function
Minor fixes