Building against the existing 3.1 libraries should continue to work, until
the precompiled libraries are committed for all platforms.
* Enable WebP by default.
* Update Windows for new library file names.
* Automatically clear outdated CMake cache variables when upgrading to new
libraries.
* Fix static library linking order issues on Linux for OpenEXR and OpenVDB.
Implemented by Ray Molenkamp, Sybren Stüvel and Brecht Van Lommel.
Ref T95206
604c33e694