simplify sprite scrolling

[john-doherty]

Sprite scrolling methods were confusing. This PR replaces parallaxScrollX, parallaxScrollY, and loopScrollX with a single scroll(speedX, speedY, options) method. Same runtime behaviour, fewer concepts.

Before:

cloud.parallaxScrollX(-0.5, 2000);
grass.loopScrollX(-2);

After:

cloud.scroll(-0.5, 0, { respawnAfter: 2000, offset: 100 });
grass.scroll(-2, 0, { loop: true });

Options:

• loop — wrap by sprite size for seamless tiled backgrounds
• respawnAfter — ms delay before reappearing on the opposite edge
• offset — max random distance beyond the edge on respawn (0 = exact edge)