Add support for a global palette.
Move the addFrame palette to the options object, as it is now optional.  You
must supply at least one palette though, but you can override a global palette
with a local palette.
This should additionally support background colors, but I am not sure how those
are actually handled by GIF viewers.
Pass through the frame x/y into the GIF.
Fix a mistake of taking the global vs local frame w/h.
Perhaps the variables should be named better.
Add a comment and check about background index behavior.
Support disposal and transparency, although disposal seems to be very buggy in viewers.
Only emit Graphics Control Extension when needed.