This code to free shaders by casting a struct to a pointer array is not valid
in general, and particularly when compiling with Clang (on Linux and macOS) it
fails and can read invalid/uninitialized memory .
This commit is a full refactor of the grease pencil modules including Draw Engine, Modifiers, VFX, depsgraph update, improvements in operators and conversion of Sculpt and Weight paint tools to real brushes.
Also, a huge code cleanup has been done at all levels.
Thanks to @fclem for his work and yo @pepeland and @mendio for the testing and help in the development.
Differential Revision: https://developer.blender.org/D6293
