* 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.
* Add an exception catcher that pops open a UI window showing errors that
occur in doodad scripts during gameplay.
* Shows a preview of the header of the error (character wrapped) with a
Copy button to copy the full raw text to clipboard for inspection.
* Buttons to dismiss the modal once or stop any further errors from
opening during this gameplay session (until next restart).
* Add developer shell commands to test the exception catcher:
- 'throw <message>' to throw a custom message.
- 'throw2' to stress test a "long" message.
- 'throw3' to throw a realistic message copied from an actual error.
* Scripting engine: console.log() and friends will now insert the script
VM's name in front of its messages (the filename + actor ID).
