Create small overlap in tiles.

Tiles overlap on the bottom line and the right column.
Having an overlap allows us to get neighbor points for each point in a
tile, regardless of its position.

TODO.md

@@ -7,16 +7,18 @@
 
 [ ] Optimization for square images?
 [X] Fixed point computation?
+[-] -funroll-loops
 
 # Cache
 [-] Rotate per channel -> no gain
 [ ] Load pixels in 64-bit variable
 [X] Cut image in tiles
- [ ] Overlap?
+ [X] Overlap
  [-] Rotate in one temp tile then copy/move it
  [X] Align tiles in memory
 [-] Align memory -> no gain
 [ ] RGBX format
+[ ] Spiral layout?
 
 # Quality
 [X] Interpolate using SIMD, SSE (no big gain)