Smart quotes.
--scale takes fractional arguments like 1/3.
Scale_context -> sctx
Resize refactoring.

- Point scaling: use pixel midpoints. See this:
  http://www.virtualdub.org/blog/pivot/entry.php?id=86
- Refactor to use doubles more frequently. This is a bit faster and
  common code is combined.
Resize frame buffers have an alpha channel.

This is roughly 10% slower than the previous code, so
perhaps I will revisit.
Add "catrom" resize method: Catmull-Rom resampling.

This actually uses the alpha channel stuff from the previous commit.
Avoid bogus warnings about redundant options.
Add 'mitchell', 'lanczos2', and 'lanczos3' filters.