- I wanted to add new parameters, and rather than having the function
  signature grow out of control, I decided to switch to the same
  `params`-style as the rest of libplacebo, for future forwards
  compatibility.

- We need to expose the identifier, because users of raw shaders might
  want to combine multiple shaders into the same GLSL program.

This kills off the hacky `_ex` functions and uses a params struct. Also
organizes some fields around, for reasons.