kernel: tweak gaussian deviation

So that the shadow doesn't look cut off or fuzzy.

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

src/kernel.h

@@ -18,8 +18,16 @@ typedef struct conv {
 double attr_pure sum_kernel(const conv *map, int x, int y, int width, int height);
 double attr_pure 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);
+/// Create a kernel with gaussian distribution with standard deviation `r`, and size
+/// `size`.
+conv *gaussian_kernel(double r, int size);
+
+/// Create a gaussian kernel with auto detected standard deviation. The choosen standard
+/// deviation tries to make sure the outer most pixels of the shadow are completely
+/// transparent.
+///
+/// @param[in] shadow_radius the radius of the shadow
+conv *gaussian_kernel_autodetect_deviation(int shadow_radius);
 
 /// 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