glsl/linker: handle errors when a variable local size is used

glsl/linker: handle errors when a variable local size is used

Compute shaders can now include a fixed local size as defined by
ARB_compute_shader or a variable size as defined by
ARB_compute_variable_group_size.

v2: - update formatting spec quotations (Ian)
    - various cosmetic changes (Ian)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>