Move image/svg handling into iced_graphics

The `TextureStore` trait is implemented by the atlas, and can also be
implemented in the glow renderer or in a software renderer.

The API here may be improved in the future, but API stability is
presumably not a huge issue since these types will only be used by
renderer backends.

graphics/src/image.rs

@@ -0,0 +1,34 @@
+//! Image loading and caching
+
+use std::fmt::Debug;
+
+#[cfg(feature = "image_rs")]
+pub mod raster;
+
+#[cfg(feature = "svg")]
+pub mod vector;
+
+/// Entry in the texture store
+pub trait TextureStoreEntry: Debug {
+    /// Width and height of the entry
+    fn size(&self) -> (u32, u32);
+}
+
+/// Stores cached image data for use in rendering
+pub trait TextureStore {
+    /// Entry in the texture store
+    type Entry: TextureStoreEntry;
+    /// State passed to upload/remove
+    type State<'a>;
+
+    /// Upload image data
+    fn upload(
+        &mut self,
+        width: u32,
+        height: u32,
+        data: &[u8],
+        state: &mut Self::State<'_>,
+    ) -> Option<Self::Entry>;
+    /// Remome image from store
+    fn remove(&mut self, entry: &Self::Entry, state: &mut Self::State<'_>);
+}