Updated toolchains submodule.
Windows build fixes (working around std::u32string bugs).
JpegImporter,PngImporter: fix Windows linker failures.

The vtables weren't exported.
JpegImporter: work around libjpeg-turbo/windows.h typedef conflict.

Both define INT32, but not to the same type.
WavAudioImporter: fix linker error on Windows.
Updated dependencies and enabled plugins in MinGW PKGBUILD.

Supported everything except plugins requiring Qt and HarfBuzz, as the
dependencies are too heavy or too obscure (no HB windows builds yet).