Add thumbnails for layers (#286)

Fixes: #285

core/document/src/layers/folder.rs

@@ -74,6 +74,10 @@ impl Folder {
 		self.layers.as_slice()
 	}
 
+	pub fn layers_mut(&mut self) -> &mut [Layer] {
+		self.layers.as_mut_slice()
+	}
+
 	pub fn layer(&self, id: LayerId) -> Option<&Layer> {
 		let pos = self.layer_ids.iter().position(|x| *x == id)?;
 		Some(&self.layers[pos])

core/document/src/layers/mod.rs

@@ -175,6 +175,10 @@ impl Layer {
 		self.data.to_kurbo_path(self.transform, self.style)
 	}
 
+	pub fn current_bounding_box(&self) -> Option<[DVec2; 2]> {
+		self.bounding_box(self.transform, self.style)
+	}
+
 	pub fn bounding_box(&self, transform: glam::DAffine2, style: style::PathStyle) -> Option<[DVec2; 2]> {
 		if let Ok(folder) = self.as_folder() {
 			folder.bounding_box(transform)