Fix bevy_ui_widgets scrollbar bug where scrollbar_size wasn't taken into account

[PPakalns]

Objective

Bevy ui widget scrollbar implementation didn't correctly handle scrollbar width inserted by taffy.

Solution

Correctly handle scrollbar width similarly as it is done in bevy engine scroll position calculation:

bevy/crates/bevy_ui/src/layout/mod.rs

Lines 334 to 336 in 0d46518

	let max_possible_offset =
	(content_size - layout_size + node.scrollbar_size).max(Vec2::ZERO);
	let clamped_scroll_position = scroll_position.clamp(Vec2::ZERO, max_possible_offset);

Testing

• In personal project added scrollareas with scrollbars that automatically attach to overflow: scroll elements and utilize space provided by taffy scrollbar-width.

[PPakalns]

Changes in action, utilizes scrollbar_size provided by taffy:

scroll-area-auto-attached.mov

[alice-i-cecile]

@rossleonardy, review? :)