Removal of offscreen display support (--offscreen)

[Alexandros Frantzis]

Hi all,

for a few years now, Mir has provided the --offscreen option to render
to offscreen buffers instead of the real outputs (note that it still
needs proper GPU/GLESv2 support, and it doesn't work in all
environments). This option was implemented experimentally in order to
help with automated QA scenarios, but it was never adopted, and we
didn't get any feedback or improvement requests.

Unless someone is currently using --offscreen, or has a valid use case
for it, I would like to remove it from our code base.

Thanks,
Alexandros