Refactor texture atlas

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

wgpu/src/texture/atlas/allocator.rs

@@ -0,0 +1,57 @@
+use guillotiere::{AtlasAllocator, Size};
+
+pub struct Allocator {
+    raw: AtlasAllocator,
+}
+
+impl Allocator {
+    pub fn new(size: u32) -> Allocator {
+        let raw = AtlasAllocator::new(Size::new(size as i32, size as i32));
+
+        Allocator { raw }
+    }
+
+    pub fn allocate(&mut self, width: u32, height: u32) -> Option<Region> {
+        let allocation = self
+            .raw
+            .allocate(Size::new(width as i32 + 2, height as i32 + 2))?;
+
+        Some(Region(allocation))
+    }
+
+    pub fn deallocate(&mut self, region: Region) {
+        self.raw.deallocate(region.0.id);
+    }
+}
+
+pub struct Region(guillotiere::Allocation);
+
+impl Region {
+    pub fn position(&self) -> (u32, u32) {
+        let rectangle = &self.0.rectangle;
+
+        (rectangle.min.x as u32 + 1, rectangle.min.y as u32 + 1)
+    }
+
+    pub fn size(&self) -> (u32, u32) {
+        let size = self.0.rectangle.size();
+
+        (size.width as u32 - 2, size.height as u32 - 2)
+    }
+}
+
+impl std::fmt::Debug for Allocator {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "Allocator")
+    }
+}
+
+impl std::fmt::Debug for Region {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "Region {{ id: {:?}, rectangle: {:?} }}",
+            self.0.id, self.0.rectangle
+        )
+    }
+}