This gives users control over where a callout should join to the label
text. (Previously, you only had control over where the callout would
join to the corresponding feature geometry).
Choices include:
- Closest point (previous behavior)
- Label Centroid
- Fixed corners: Top left/top right/bottom left/bottom right/etc
Data defined control over the label anchor is also possible
which controls a distance from the feature where callout lines end
Makes for visually more appealing callouts vs drawing them right
up against the edges of features
...instead of being rendered on top of some. For this callout style,
we don't want callouts overlapping labels (rather the opposite). But
leave API in place to allow other callout styles to render below
their associated labels only, as this may be wanted for some styles
(e.g. balloon style callouts)
