* Fix collision detection to allow actors to walk up slopes smoothly, without
losing any horizontal velocity.
* Fix scrolling a level canvas so that chunks near the right or bottom edge
of the viewpoint were getting culled prematurely.
* Centralize JavaScript exception catching logic to attach Go and JS stack
traces where possible to be more useful for debugging.
* Performance: flush all SDL2 textures from memory between scene transitions
in the app. Also add a `flush-textures` dev console command to flush the
textures at any time - they all should regenerate if still needed based on
underlying go.Images which can be garbage collected.
Noah
6def8f7625