Refactor shadow generation

Trying to make the code easier to understand. The logic is unchanged.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/kernel.h

@@ -14,6 +14,11 @@ typedef struct conv {
 /// Calculate the sum of a rectangle part of the convolution kernel
 /// the rectangle is defined by top left (x, y), and a size (width x height)
 double attr_const sum_kernel(const conv *map, int x, int y, int width, int height);
+double attr_const sum_kernel_normalized(const conv *map, int x, int y, int width, int height);
 
 /// Create a kernel with gaussian distribution of radius r
 conv *gaussian_kernel(double r);
+
+/// preprocess kernels to make shadow generation faster
+/// shadow_sum[x*d+y] is the sum of the kernel from (0, 0) to (x, y), inclusive
+void shadow_preprocess(conv *map, double **shadow_sum);