directshow plugins: add meson build?

The directshow-based plugins are currently not built in the autotools build. Some of them have cmake build files. These plugins rely on base classes from an external dshow sample repository which needs to be built separately first. Those samples are MIT licensed these days, so we might be able to ship them if we wanted to.

• dshow baseclasses - wip meson subproject (symbol exports still need fixing)
• sys/dshowsrcwrapper
• sys/dshowdecwrapper
• sys/dshowvideosink (remove this? no autotools nor cmake build, unclear if anyone actually uses this?)