Support segment mode in Grease Pencil v3 selection operators.
This is a continuation of #109221.
Segment selection works in multiple stages:
1. When enabled, the 2D evaluated lines for all visible curves are
entered into a BVH tree. This tree is used to find intersections later
on. All the visible drawings are in the tree, an `OffsetIndices` array
is used to record ranges of BVH elements for each drawing.
2. Primary selection functions get alternative versions that produce an
`IndexMask` instead of writing directly to selection attributes. This
makes it possible to detect select actions on points that are already
selected. Using a delta of selection attributes for this is not enough
to detect such user actions (and inefficient).
3. In segment mode the `IndexMask` is extended to fully cover segment
points (or exclude them when deselecting respectively). This first
performs and intersection test using the BVH tree, then finds all point
range with selected elements.
4. Finally the extended index mask is applied to selection attributes as
usual.
Pull Request: https://projects.blender.org/blender/blender/pulls/126027
2e9fc8ef2d