For tree support, use lslices as tree support island when generating brim,
as this is faster and more accurate.
For normal support, still use "support_fills.polygons_covered_by_spacing()" as support island when generating brim;
Jira: studio 4332
Change-Id: Ibfadd3a166606f824e5780b57112fff221470aaf
(cherry picked from commit 64960b19818c7029eaaaf3d8a89804aeaa26f11d)
(cherry picked from commit 181b05c236)