Use a unique buffer for pixels (scanline).

- Add rotate_pixel().
- Remove APoint, use Point instead.

TODO.md

@@ -6,3 +6,12 @@
 [ ] Fix out-of-bounds pixel set
 
 [ ] Optimization for square images?
+[ ] Fixed point computation?
+
+# Cache
+[X] Rotate per channel -> no gain
+[ ] Load pixels in uint64-t
+[ ] Cut image in tiles
+
+# Quality
+[ ] Interpolate using SIMD, SSE