Shader API refactoring

[guaje]

The addition of new functions to our shader API (here) allow for a dynamic and more flexible way to reuse shader functions.

Some parts of the codebase are already using the new system, for instance:

• Principled material (here).
• SDF and Ray Marching tutorial (here).
• Billboard implementation (Cylindrical billboard implementation #677).

This issue is created to keep track of the upcoming changes in order to have consistency in the way we add and support custom shaders in FURY. The following is a list of the major functionalities/shaders and progress in refactoring them according to the new API:

• Billboard functions (billboard_dec.frag, billboard_dec.vert, billboard_impl.frag, billboard_impl.vert, and billboard_spheres_impl.frag). Partially solved by Cylindrical billboard implementation #677.
• Geometry shader for lines (line.geom).
• Marker functions (marker_billboard_dec.frag, marker_billboard_dec.vert, marker_billboard_impl.frag, and marker_billboard_impl.vert).
• Peak actor functions (peak_dec.frag, peak_dec.vert, peak_impl.frag, and peak_impl.vert). Solved in Update Peak actor shader #767
• SDF demo functions (sdf_dec.frag, sdf_dec.vert, sdf_impl.frag, sdf_impl.vert, sdfmulti_dec.frag, and sdfmulti_impl.frag).

[skoudoro]

Thanks for writing this issue @guaje. Exciting refactoring!