Refactor texture atlas

- Split into multiple modules
- Rename some concepts
- Change API details

wgpu/src/texture/atlas/layer.rs

@@ -0,0 +1,17 @@
+use crate::texture::atlas::Allocator;
+
+#[derive(Debug)]
+pub enum Layer {
+    Empty,
+    Busy(Allocator),
+    Full,
+}
+
+impl Layer {
+    pub fn is_empty(&self) -> bool {
+        match self {
+            Layer::Empty => true,
+            _ => false,
+        }
+    }
+}