currently the spirv spec says that subgroups are restricted to kernels. however glsl supports subgroups in regular compute shaders. seems like the glsl spec team forgot to tell the spirv spec team to update their specs. this leads to spirv-tools complaining about an invalid decorator in the opt/validation phases which in turn leads to rust-gpu not being competitive with glsl for no real reason. removing the dependency on spirv-tools would allow supporting "defacto" standards instead of just what was bothered to be written down in a spec.

• [0] https://github.com/KhronosGroup/GLSL/blob/master/extensions/khr/GL_KHR_shader_subgroup.txt