The default value is 1, and the assumption in target_spirv is that if
the first one is set, all are set, but the source code doesn't need to
set any explicitly.
This means that `layout(triangles)` etc now work, though there are
issues with frag shaders missing functions (since getting function calls
working) and compute shaders silently failing.
Attributes seem appropriate as GLSL's qualifiers affect variables rather
than types (since there's no typedef).
Not much is done with the attributes yet other than some basic error
checking (duplicates of non-layout attributes) and debug output, but
most (if not all) declarations get to the declaration code with
attributes intact.
