This patch adds more control to existing Envelope modifier:

• offset start, end and general offset for generated lines, especially noticeable when using skip strokes
• material offset now has end material parameter which creates range of materials
• material alteration is set in step parameter

known issues for the first iteration:

• limited support for offsetting parameters in non cyclic strokes, only cyclic fills has full implementation
• material indexing does not respect -1 the way it did before, has to be fixed
• crashes in some cases when setting end material (best to just type in 5 or higher to avoid crashes)

This iteration is meant for some people to play around and come up with some artistic examples for it.
It is not practical to have so many parameters exposed in one panel, would like to discuss how to best split it.