Canvas.update allows one arg. FPS is displayed nicer.

Because Canvas.update did not have any args, it could
not be used as an event handler. Now it can, which makes
attaching e.g. a timer to it much easier/cleaner.
Remove canvas.update from on_draw on all examples.

Use timer instead.
flake
add docs to canvas.update
Make examples use timer more effectively

Made some examples use 60 Hz
Make examples use Timer('auto', ...)
Better canvas.measure_fps
flake
Merge pull request #454 from almarklein/examples-60-fps

Let all dynamic examples run at 60 fps
Cleaning up transform system
rename SceneCanvas.pixels => SceneCanvas.entity
Change "pixel" to "canvas" in SceneEvent
Partial implementation of entity_transform
Add InverseTransform
fixes; scene examples working again
Fixed visual examples
Grid visual works; some artifacts in polar plot.
Reversed grid y-orientation; cell (0, 0) is at top-left.
Made more interesting transform in grid example polar cell.
Avoid bright spots at grid line intersections.
Clean up docstrings, remove old code
fix document pop on entity pop