boost speed of bucket in the pressence of selection by about 87%

[Variable-ind]

This introduces two independent optimizations:

1. lock_selection_rect optimization:

Stores existing selection rect as cache and forces get_selection_rect in SelectionMap to use cache instead. use this to save time when the rect isn't meant to change between repeated calls (e.g see it's use in Bucket tool _flood_fill). This causes roughly an 87% boost to the area fill algorithm

2. area_fill compatible with selection size:

when segments were created in _compute_segments_for_image

for j in image.get_height():
	_add_new_segment(j)

When we have selection, this ends up adding segments not necessary for calculation. As an optimization, i change it to

var y_range = [0, image.get_height()]
	_area_end_idx = project.size.y - 1
	_area_start_idx = 0
	if project.has_selection:
		var selection_rect := project.selection_map.get_selection_rect(project)
		_area_start_idx = selection_rect.position.y
		_area_end_idx = selection_rect.end.y - 1
		y_range = [selection_rect.position.y, selection_rect.end.y]
	for j in range(y_range[0], y_range[1]):
		_add_new_segment(j)

This dramatically decreases the fill time even further.

Benchmark of area_fill on a 30x30 selection in a 646x646 canvas

Version	Area fill time (msec)
master	3834
PR	32

Related optimization that tagged along.

• The lock_selection_rect optimization is used by draw_tool and draw_fill_gap as well (this will be noticeable when drawing a line using Pencil on a 5000x5000 canvas in presence of selection).

[OverloadedOrama]

Looks good to me, thank you!