Rather than fixing/determining the pl_glsl_version at compiler creation
time, this is now taken dynamically as a compilation parameter. In
addition to this, the exact API / target version to compile against will
be inferred from the GLSL description, rather than set explicitly. This
allows us to take into account some of these limits during shader
compilation.

Move stuff into the common glsl/ subdir, mostly to allow the creation of
glsl/utils.h which contains shared helpers between C and C++ that can't
be placed into spirv.h directly due to C99/C++ incompatibility.

Finally, rename some usage sites for consistency, enabling `stage` to
always refer to the shader stage, `shader` to refer to the shader
itself, and `glsl` to refer to the pl_glsl_version struct.

Requires bumping the shaderc dependency. Fortunately, none of the usual
candidates (debian stable etc.) even *package* ubuntu, so we're fine.