sample/test code: use the plain container to keep track of x/y/w/h of a 3d draw region

[galaxyjim]

write a sample....

using clipping and pyglet open gl commands
an application can draw directly to any region on the screen

the plain container could be used to allow keep track of a screen region.

this is an alternative to having the opengl code always

behind the widgets, or
in a fixed screen location.

depends on gh-26

[galaxyjim]

test4.py

doesn't use clipping, but does draw using the canvas info.