It seems like OSX drivers are using standard attributes for passing
gl_VertexID and gl_InstanceID to the vertex shader, and count them in the
limit of MAX_VERTEX_ATTRIBS.
This patch make sure to never use more than 13 attributes by packing some
attributes together.
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
