Making a fast API compatible way to copy image data.

This can improve on the python array_to_image function
where we already have an allocated image struct and simply
need to copy the data into the correct format/shape without
reallocating.

Aside: I think array_to_image does two frees because the pointer
data_as returns doesn't own the memory that is ultimately freed
in free_image.

src/image.c

@@ -2137,3 +2137,22 @@ void free_image(image m)
         free(m.data);
     }
 }
+
+// Fast copy data from a contiguous byte array into the image.
+LIB_API void copy_image_from_bytes(image im, char *pdata)
+{
+    unsigned char *data = (unsigned char*)pdata;
+    int i, k, j;
+    int w = im.w;
+    int h = im.h;
+    int c = im.c;
+    for (k = 0; k < c; ++k) {
+        for (j = 0; j < h; ++j) {
+            for (i = 0; i < w; ++i) {
+                int dst_index = i + w * j + w * h*k;
+                int src_index = k + c * i + c * w*j;
+                im.data[dst_index] = (float)data[src_index] / 255.;
+            }
+        }
+    }
+}

src/image.h

@@ -33,6 +33,7 @@ image random_crop_image(image im, int w, int h);
 image random_augment_image(image im, float angle, float aspect, int low, int high, int size);
 void random_distort_image(image im, float hue, float saturation, float exposure);
 //LIB_API image resize_image(image im, int w, int h);
+LIB_API void copy_image_from_bytes(image im, char *pdata);
 void fill_image(image m, float s);
 void letterbox_image_into(image im, int w, int h, image boxed);
 //LIB_API image letterbox_image(image im, int w, int h);