glsl/cs: Initialize gl_LocalInvocationIndex in main()

glsl/cs: Initialize gl_LocalInvocationIndex in main()

We initialize gl_LocalInvocationIndex based on the extension spec
formula:

    gl_LocalInvocationIndex =
        gl_LocalInvocationID.z * gl_WorkGroupSize.x * gl_WorkGroupSize.y +
        gl_LocalInvocationID.y * gl_WorkGroupSize.x +
        gl_LocalInvocationID.x;

https://www.opengl.org/registry/specs/ARB/compute_shader.txt

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>