View2DScrollers used the memory manager to allocate memory. This isn't a
problem but in a upcoming change the scrollers will be drawn more often
than it used to (See {D8066}). To limit the number of allocations and
frees this patch will use the stack for allocation.
Reviewed By: Campbell Barton
Differential Revision: https://developer.blender.org/D8076
2fdca5bd19