Accept a tile halo that the frame's edge cuts short

The test demanded a full halo on every side except the frame's own
edge, but a tile one step in from it is grown to the edge and no
further, exactly where the untiled render stops too.

core/dr-pipeline/src/tiles.rs

@@ -139,11 +139,12 @@ mod tests {
             assert!(gw <= max && gh <= max, "{t:?} does not fit");
             assert_eq!(gx % TILE_ALIGN, 0, "{t:?} starts off the grid");
             assert_eq!(gy % TILE_ALIGN, 0, "{t:?} starts off the grid");
-            // The halo is there on every side the frame does not end.
-            assert!(kx == 0 || kx - gx >= halo);
-            assert!(ky == 0 || ky - gy >= halo);
-            assert!(kx + kw == frame.0 || gx + gw - (kx + kw) >= halo);
-            assert!(ky + kh == frame.1 || gy + gh - (ky + kh) >= halo);
+            // The halo is there on every side, or the frame ends first — in
+            // which case the untiled render stops at the same edge.
+            assert!(gx == 0 || kx - gx >= halo);
+            assert!(gy == 0 || ky - gy >= halo);
+            assert!(gx + gw == frame.0 || gx + gw - (kx + kw) >= halo);
+            assert!(gy + gh == frame.1 || gy + gh - (ky + kh) >= halo);
         }
     }