Improve custom vignettes in a few ways:

• Allow customizing the audio (currently still defaults to the prologue audio for backwards compatibility).
• No longer ends after a hardcoded wait duration, uses the CoreVignette approach instead.
• Allow customizing wait times at the start and end (vanilla has a shorter initial wait time in CoreVignette than IntroVignette, the wait time at the end can be used e.g. to wait for a sound to finish as IntroVignette does).
• Make sure the snow direction always gets applied, not just when reusing an existing HiresSnow (probably never matters).
• Center all text lines for text with line breaks.
• Make snow in vignette blow to the left by default to match vanilla.

The new wait time attributes, as well as the wind direction one, should also be documented on the wiki.