feat(windows): OSKBulkRenderer feature suggestions

This is very useful! Some suggestions for future bells & whistles:

1. There's probably some .kvk files hidden in legacy/**.kmp packages (e.g. thompsonu). It would be good to render those too.
2. Are there console warnings if a .png file failes to generate?
3. tofu-detection would be nice :)

Additional:
4. Add notes on font used for rendering
5. Find fonts associated with OSK and provide those for embedded font render to reduce tofu.

_Originally posted by @darcywong00 in https://github.com/keymanapp/keyman/pull/1428#issuecomment-447195555_