Convert TIFF images to PNG.

Bug: 70866
Change-Id: I933e3cac2b8cae8a31759bcaeefed2703b84c338
Handle animated GIFs; remove `easyimage` dependency.

We don't really need to pull in the `easyimage` dependency when we
just need to invoke `convert` from imagemagick, and the easyimage
package doesn't provide a clean way to pass options to convert.  (Its
argument quoting was also suspect when I looked at the source.)  So
remove the `easyimage` dependency and manually pass a `-delete` option
in order to remove all but the first frame of an animated gif.
Otherwise `convert` would spew multiple `<filename>-%0d.png` files all
over the output directory.

Note that imagemagick also has a bunch of weird filename handling,
mostly set off by colons in the input filename and percents in the
output filename.  Try to protect ourself better.

While I was added, made the figure handling a little more robust by
explicitly checking that all required files are present before
emitting LaTeX which mentions them.

Change-Id: I70a9a6a08cc3f65e775f872728e2e90097fda4b3