[FEATURE REQUEST] Make finding ptex more flexible

**Is your feature request related to a problem? Please describe.**
The available options for finding ptex can be too rigid for some setups. The issue stems from` PTEX_LOCATION` being used for both headers and libraries, with additions also being added to any path being passed in.

If `PTEX_INCLUDE_DIR` is passed to cmake, it gets overridden by `find_path()`, same with passing `PTEX_LIBRARY`.

**Describe the solution you'd like**
I would suggest a similar approach to that used in [FindFFmpeg](https://github.com/OpenImageIO/oiio/blob/master/src/cmake/modules/FindFFmpeg.cmake#L23), checking for `PTEX_INCLUDE_DIR` and `PTEX_LIBRARY` and only searching if these are not set.

Start by locating the CMake logic that finds ptex and compare its behavior with the linked FindFFmpeg.cmake approach. Verify the PTEX_INCLUDE_DIR and PTEX_LIBRARY inputs are preserved and that searching occurs only when those variables are unset; done means explicit paths are no longer overridden.

Written by the indexing model from the issue text.