Please update the Windows build instructions for libaom to use vcpkg system

[paul-gruzdev]

Run the aom.cmd script in the third-party directory to download libaom and compile it.
This script requires cmake, ninja nasm to be installed, and even then it won't build the libaom successfully. In order to build it, use:

vcpkg install aom:x64-windows

command in vcpkg folder. Also make sure you do this before running vcpkg install libheif because the latter should locate the aom build before building itself.

[bradh]

Possibly you could propose the changes you think are appropriate (either in place of, or along side, the existing instructions) as a Pull Request. That would ensure your needs are addressed, and make it easy for the maintainers to apply.

You can just do this on github if it is easier.