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.

Makefile

@@ -1,15 +1,16 @@
 CXX     = clang++
 CXXFLAGS = -std=c++11 -W -Wall -O3 -ffast-math -Werror -g
 BUILD_DIR=/tmp
+IMG=img/lena.ppm
 
 all: rotation.cpp
 	$(CXX) $(CXXFLAGS) $< -o $(BUILD_DIR)/rotation
 
 clean:
-	@rm -f *~ *.o .*.swp *.ppm
+	@rm -f *~ *.o .*.swp *.ppm cachegrind.out.*
 
 run: all
-	$(BUILD_DIR)/rotation img/lena.ppm
+	$(BUILD_DIR)/rotation $(IMG)
 
 cachegrind: all
-	valgrind --tool=cachegrind $(BUILD_DIR)/rotation img/lena.ppm
+	valgrind --tool=cachegrind $(BUILD_DIR)/rotation $(IMG)