Generalized vertex dispatch, nuklear UI sample

This MR adds a number of features needed to get a basic nuklear UI running on libplacebo. In particular, this includes the ability to dispatch arbitrary vertices using the pl_shader infrastructure, as well as the ability to dispatch indexed draw calls.

For now, the demo does nothing except allow you to change the window background color. I plan on adding example code to hook up all of pl_render_params to the nuklear UI.