[2.9] Versatile clipping for lines

[kurkle]

This PR changes the default clipping so it will clip at chartArea edge, if the scale defining that edge is limited at that end (by ticks.min/ticks.max). If min/max is not defined, the current clipping rules apply (borderWidth / 2 outside chartArea)

Overriding this clipping is also allowed per dataset:

clip: 10 // would clip 10px outside chartArea on each edge
clip: -10 // would clip 10px inside chartArea on each edge
clip: false // would not clip
clip: 0 // would clip to chartArea
clip: { left: 10, right: -10, top: false, bottom: 0) // per edge config

Fixes: #6631