Allow avoid merging layers with pen tool

[0HyperCube]

Reproduction

When drawing with the pen tool, a user may wish to keep two layers separate. This could be useful for e.g. applying different fills or animations. However if the anchor shares a position with an anchor from another layer, it will always automatically merge.

don.t_merge.mp4

Fix

• Indication when placing the anchor will automatically merge layers (e.g. in the tooltip). It is currently very hard to tell.
• Option to avoid merging the layers even when they have the same anchor position.

Code

Graphite/editor/src/messages/tool/tool_messages/pen_tool.rs

Lines 1883 to 1900 in cc7d700

	// Merge two layers if the point is connected to the end point of another path
	// This might not be the correct solution to artboards being included as the other layer,
	// which occurs due to the `compute_modified_vector` call in `should_extend` using the click targets for a layer instead of vector.
	let layers = LayerNodeIdentifier::ROOT_PARENT
	.descendants(document.metadata())
	.filter(|layer| !document.network_interface.is_artboard(&layer.to_node(), &[]));
	if let Some((other_layer, _, _)) = should_extend(document, viewport_vec, crate::consts::SNAP_POINT_TOLERANCE, layers) {
	let selected_nodes = document.network_interface.selected_nodes();
	let mut selected_layers = selected_nodes.selected_layers(document.metadata());
	if let Some(current_layer) = selected_layers
	.next()
	.filter(|current_layer| selected_layers.next().is_none() && *current_layer != other_layer)
	.or(tool_data.current_layer.filter(|layer| *layer != other_layer))
	{
	merge_layers(document, current_layer, other_layer, responses);
	}
	}