fix markers on lines with variable aesthetics

[Fil]

for lines with variable aesthetics we want to maintain the higher-level semantics of markers:

• markerStart matches the start of the line
• markerMid matches the points which are not at the start or the end
• markerEnd matches the end of the line

Since these lines are implemented as multiple paths, we need to change the low-level implementation of markers:

• markerStart only applies to the first segment of a line
• markerMid applies to all the segments, complemented by the start of all but the first segments
• markerEnd only applies to the last segment of a line

closes #2093