Windows CI: build dependencies ourselves

[lgritz]

Last week, Vcpkg apparently had some flaw that recently caused it to
fail when building/installing libpng (among other things). It's been
fixed in the vcpkg main branch, but that hasn't propagated to the
vcpkg that's installed on the GitHub Actions CI runners, and that's
causing our Windows CI tests to fail.

Rather than wait out the fix, I'm just taking it into my own hands by
building all our dependencies from source on Windows. This means adding
bash scripts to download and build zlib, libpng, and libjpeg-turbo (and
maybe those will be helpful to others as well).

Late breaking news They beat me to it, vcpkg has been updated and
the GitHub CI Windows tests are passing again.

But I had already done this work! So I still would like to switch to
this methodology because:

• Sometimes it's faster to build ourselves rather than have vcpkg
  build, since vcpkg always builds both release and debug, but we only
  need the release builds of the dependencies.

• This makes it easier to customize the specific version of a
  dependency, or particular build options, which are hard or
  impossible to change for a vcpkg build.

• These same dependency-building scripts could be useful for people who
  are not on Windows or not interested in using vcpkg.

Also some minor changes to the build_libtiff and build_openexr scripts
to synchronize some of the idioms I use in these scripts.

Rename workflow.yml to ci.yml to adhere to convention and also make it
so we can have multiple workflows in the future.

Signed-off-by: Larry Gritz lg@larrygritz.com