shaders: add pl_shader_info to replace pl_dispatch_info.shader

The pl_dispatch_info had a number of serious shortcomings. One of these was the fact that it referenced pl_shader_res, which was not only preventing me from deprecating/replacing this API internally, but more importantly, was clumsy and required users to make manual deepcopies of this struct.

Replace it by a new pl_shader_info, which is also refcounted, to allow users to pass it around more easily.

As an aside, this commit fixes a bug where the steps array would sometimes contain NULL entries as a consequence of the way the shader description was generated.