New: games/vkquake - fork of quakespasm using Vulkan API

Hi,

Please find attached a port of vkQuake. It shares many similarities
with games/quakespasm which it is based on; except for the use of
Vulkan as the graphics API.

There are 2 reasons to add this as yet another quake port:

1. Potential performance advantage over quakespasm (I didn't do
performance comparison).
2. As a test application for vulkan API that is more complex than
vkcube and may cover more of the API.

Like quakespasm, it shows the following messages during runtime:

[...]
couldn't exec config.cfg
couldn't exec autoexec.cfg
[...]
Couldn't write config.cfg.
[...]

I left this alone for now as the same messages appear in quakespasm and
the game runs otherwise.

ok?