tiling: Store tiling for each layer and fix bugs

Tilings now have a flexible origin and are stored with the
tile layer. This allows to make the first tile centered
in the viewport, allowing for faster getting a complete
viewport.

This also fixes some rendering bugs caused by forgetting
tiles the were still needed. But not all of them.

Fixes #83, #94
Issue #82

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/179>

src/decoder/mod.rs

@@ -27,6 +27,7 @@ pub use formats::{ImageFormat, RSVG_MAX_SIZE};
 /// This happens for initial loading or when zoom level or viewing area changes.
 pub struct TileRequest {
     pub viewport: graphene::Rect,
+    pub area: graphene::Rect,
     pub zoom: f64,
 }