Generalized Makefile a bit.  Should probably shift to a configure script soon.
Refactored code to make it a little easier to add new variants.
Code cleanup.
Checkpoint.  Got circles drawing on the screen.
Added soe.rs to demo driver.
Make soe demo by default for now.
Factored new_shape and new_circle out into fn's.
Moved flag settings out to static constants.
Revised code so that the moving shapes can have independent velocities.
Made actual Shape struct to carry the width/height of surface in visible way.
Some minor code cleanup.
Experimentally reduce radius to 10 to illustrate bug in current code.
Take optimized builds out of Makefile for now.
Alpha-rename Moving trait to Bouncing, since the behavior is built-in.
Add trait, fix bug.
Demonstrate trait impl dispatch.
Factored erasing code out into its own method on Bouncing.
Factored bouncer bliting into its own method.
refactored code to use loops over shape vector.
Add ability to add new balls dynamically from mouse clicks.