Release v4.28.2

[github-actions]

Sprite (and therefore AnimatedSprite) can now be constructed and used without a running Window/Applet, which previously crashed the whole JVM via System.exit().

• Root cause: Sprite()'s constructor builds a Text, whose constructor eagerly loaded the library's default font through the Processing asset pipeline. With no live Applet, that load failed and the asset-error reporter called System.exit(). Font now defers loading the underlying font file until it is actually needed (the first getFont() call during rendering), instead of doing it synchronously in its constructor. This matches how the library already preloads assets in the background once a Window is running (Applet.loadAssets()), and doesn't change behavior for normal (non-test) usage.
• Sprite.ifOnEdgeBounce() used to throw a NullPointerException if called before the sprite was added to a stage (it read stage border fields unconditionally). It's now a no-op in that case, consistent with how other stage-dependent drawing code already guards on stage == null.

Together these make it possible to unit test a Sprite subclass's movement/geometry logic (and now run(), as long as it doesn't touch other stage-dependent queries) without spinning up a real window - which is exactly what the newly added SpriteTest, AnimatedSpriteTest, and FontTest do.

Note: sprites with costumes still require a live Applet to construct, since loading actual image pixel data (width/height) is inherent to adding a costume - this only unblocks bare Sprite/AnimatedSprite construction and costume-free logic.