Expose meshface shader to UI

[froody]

In my usecase I load .ply (polygons) and .las files in the same view, but the polygons are rendered with meshface.glsl which is special-cased and not supported by the "Shader Parameters" UI. I'd love to be able to attach a UI element to the z-position of the mesh vertex shader to move polygons up and down to see where they intersect with the pointcloud.

[c42f]

Seems reasonable enough to me. Perhaps a minimal version of this could be:

• Make it possible to open shader editor windows for all the shaders in use
• Make it so that the shader parameters UI is a big list of parameters shared by all the shaders

In a sense, user-defined shader programming isn't ideally suited to adding arbitrary geometric transformations because the CPU-side code can't reason about the location of the geometry relative to the camera (so can't do culling, etc). But luckily displaz is quite naive about meshes and leaves all culling to the GPU for anything which isn't a point cloud. So this should work ok for you, I think.