replace sdl mixer/image version checking with something more robust

rewritten FindOggVorbis.cmake

fail if sparkle not found consciously

oups

rework FindSparkle.cmake

use opacity to indicate thawing. not totally happy w/ effect, but argument was made to indicate some progress towards thaw. drop "exit" since it really wasn't helpful, and not rendering tags, was more annoying than fun.

At multiple requests, change ice to be turn based. This is a more modest adjustment of existing code, that can be fairly easily flagged or reverted if desired. It also allows scripts to extend freezing, and preserves fire melting.