Unless you're very familiar with `IndexRange`, it's often hard to know what
e.g. `IndexRange(10, 15)` means. Without more context, one could think
that it means `10-14`, `10-15` or `10-24`. This patch adds named constructors
to `IndexRange` to make the behavior more obvious when writing and when
reading the code. With those one can use `IndexRange::from_begin_end(10, 15)`,
`IndexRange::from_begin_end_inclusive(10, 15)` or `IndexRange::from_begin_size(10, 15)`
respectively. While being a bit more verbose, the explicitness makes code easier to
understand and also allows abstracting away some common index computations.
The old unnamed constructor that takes a begin and size is not removed by this patch,
as that would make the patch significantly bigger. I think it's reasonable to generally
use the named constructors going forward and to change the existing usages of the
old constructor over time.
Pull Request: https://projects.blender.org/blender/blender/pulls/118606
50709ca253