Add a frame viewer

[beaugunderson]

Removes the dependency on opencv from the renderer in favor of TKinter (which is included with Python) and adds a new dependency on Pillow.

The frame viewer automatically updates for each frame and can be disabled by invoking a script like python bubbles.py --no-viewer.

[yuanming-hu]

Thank you very much! Previously I investigated several possible GUI frameworks including TKinter, and Kivy.

In the future, I think we are going to need an interactive GUI for parameter tuning (for example, in rendering, adjustment of exposure, gamma, color temperature, tint, e.t.c., please see python/taichi/gui/image_viewer.py).

The downside of Kivy is that we are going to need OpenGL ES as a new dependency. However, I guess OpenGL dep. will be added sooner or later, since rapid preview via real-time rendering is necessary. Kivy also has lots of choices of sliders which I think suits better with Taichi.

The TKinter image viewer is definitely useful, since installation of Kivy is more complicated than TKinter. For users who do not want to install Kivy (especially for whom who CANNOT use OpenGL, like in a VM), it's more convenient. Thanks. :-)

[yczhang89]

I still recommend lovecat (which I made; see https://github.com/CoffeeKitty/lovecat) for parameter tuning purposes:

• lovecat is web-based (probably a good match with Browser viewer #18);
• powerful to a Python programmer (instead of having a set of hard-coded parameters to tune, lovecat presents more possibilities);
• reduce GUI work (we could write less GUI-related code to cover the same functionalities).

lovecat currently only has a lua backend. I plan to add more backends, e.g. Rust and Python.