Refactor texture atlas

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

wgpu/src/texture/atlas/entry.rs

@@ -0,0 +1,25 @@
+use crate::texture::atlas;
+
+#[derive(Debug)]
+pub enum Entry {
+    Contiguous(atlas::Allocation),
+    Fragmented {
+        size: (u32, u32),
+        fragments: Vec<Fragment>,
+    },
+}
+
+impl Entry {
+    pub fn size(&self) -> (u32, u32) {
+        match self {
+            Entry::Contiguous(allocation) => allocation.size(),
+            Entry::Fragmented { size, .. } => *size,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Fragment {
+    pub position: (u32, u32),
+    pub allocation: atlas::Allocation,
+}