Add x264 wrap and some qt5 plugin fixes

This has been tested on Linux, macOS, and Windows (MSVC).

commit 705c106d:

meson: Add a subproject wrap file for x264
This allows building x264enc from -ugly on all platforms.

commit 5d4e7169:

gst-uninstalled: Add qt5 DLLs to PATH on Windows
This is needed for using the qmlgl out of the box inside
gst-uninstalled. Of course this won't work if the user is using
a native file to specify the path to `qmake`.

commit b27cbf87:

README.md: Document how to build the qt5 plugin