testtex --interpmode and --mipmode, and tests thereof

 Make testtex able to force interp and mip modes. Add testsuite tests to test each one individually (other than the defaults, which are covered by existing tests).

This makes it easier to debug the texture system after changes -- if just one of these tests breaks, it's easy to know we've introduced a bug in a specific part of the code that deals with that case.

Along the way, also modified TIFF output to make zip the default compression. It seems nearly always better than the old default of LZW. The original reason for favoring lzw -- that zip wasn't as widely supported in other software reading TIFF files -- is probably no longer important, many years have gone by since that decision.