Support animation-enabled devices

[klausbrunner]

Adds support for passing device functions directly to animate(), enabling rendering to animated formats without intermediate files. This can be tested with the animated webp device introduced in ragg 1.5.0.

A bit of an experiment so far but seems to work well in my tests.

# Existing approach with intermediate files (should work as before)
animate(p, device = "png", renderer = gifski_renderer())

# Direct animated device rendering
animate(p, device = ragg::agg_webp_anim, extension = ".webp")