Incremental resource loading

[jedeen]

Context

Currently, Excalibur loads all resources at once (usually before the game starts).

Proposal

Implement incremental loading, so that a subset of resources can be loaded at any particular time (for example, at the beginning of a each game level, or on an as-needed basis).

[github-actions[bot]]

This issue hasn't had any recent activity lately and is being marked as stale automatically.

[eonarheim]

Implemented!