Prevent debug error on compute shader creation

[justincredible]

Resolves #1718.

The Default derivation is justified by the trait implementation for HashMap. There is a slight readability improvement here, but replacing the remaining lines that use HashMap::with_hasher(Default::default()) would slightly degrade readability.

This approach to determining whether a program contains a compute stage is not valid generally, but it works for glium because the shader stages are partitioned into the two program types.