Refactor texture atlas

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

wgpu/src/texture/atlas/allocation.rs

@@ -0,0 +1,35 @@
+use crate::texture::atlas::{self, allocator};
+
+#[derive(Debug)]
+pub enum Allocation {
+    Partial {
+        layer: usize,
+        region: allocator::Region,
+    },
+    Full {
+        layer: usize,
+    },
+}
+
+impl Allocation {
+    pub fn position(&self) -> (u32, u32) {
+        match self {
+            Allocation::Partial { region, .. } => region.position(),
+            Allocation::Full { .. } => (0, 0),
+        }
+    }
+
+    pub fn size(&self) -> (u32, u32) {
+        match self {
+            Allocation::Partial { region, .. } => region.size(),
+            Allocation::Full { .. } => (atlas::SIZE, atlas::SIZE),
+        }
+    }
+
+    pub fn layer(&self) -> usize {
+        match self {
+            Allocation::Partial { layer, .. } => *layer,
+            Allocation::Full { layer } => *layer,
+        }
+    }
+}