TODO

turtle outlines, either 
a) redraw turtle vertices with GL_LINE_LOOP (simple but slow)
b) use texcoords/fragment shader to color edges (how to control thickness?)

camera/perspective transform and controls
 - partially done, need to do rotation

per turtle z-buffer for ordering indepentent of shape buffer draw order

drawing lines
 - list of vertex pairs + color(s) added to new *vertex* buffer
 - added per tick by engine as turtle moves
 - draw as single glDrawArrays GL_LINE call, new vao
 - not per turtle shape

stamping
 - added by turtle interface via engine to new buffer
 - same turtle geometry/shaders, different turtle buffer/vao
 - make a copy (freeze) of current turtle data into buffer
 - need to track ownership so can be cleared
 - draw with glDrawElementsInstanced


textured turtle images

load complex 3d mesh

use transform feedback to buffer to calculate simulation tick

defer turtle moving/removal until render, and do a whole array compaction, to
perform better when multiple turtles are (re)moved in a frame