Shaders can be built as spv files and installed into
$libdir/quakeforge/shaders or as spvc files and compiled into the
engine. Loading supports $builtin/name to access builtin shaders,
$shader/path to access external standard shaders and quake filesystem
access for all other paths.
This paves the way for clean initialization of the Vulkan renderer, and
very much cleans up the older renderer initialization code as gl and sw
are no longer intertwined.
