Fix Windows builds and tests

* Make math library optional.
As the comment says, it should be optional on Windows and Haiku, but the check defaults to required.
* Don't make `unistd.h` required.
* Disable transparency plugin build as it's very busted.
* Add missing headers to fix compile on Windows.
* Use a fixed pref key for test plugin loading instead of `TEST_DATA_DIR`.
The latter is an absolute path, and thus unsuitable as a preference key, since it will not start with `/` on Windows.
* Add a missing pref initialization on Windows.

Testing Done:
Compiled and run tests on Windows.

Reviewed at https://reviews.imfreedom.org/r/308/