Replace copy_memNxM functions with a generic copy/avg function.

Change-Id: I3ce849452ed4f08527de9565a9914d5ee36170aa
diff --git a/test/convolve_test.cc b/test/convolve_test.cc
index fd2bd36..6e5002f 100644
--- a/test/convolve_test.cc
+++ b/test/convolve_test.cc
@@ -22,8 +22,8 @@
 }
 
 namespace {
-typedef void (*convolve_fn_t)(const uint8_t *src, int src_stride,
-                              uint8_t *dst, int dst_stride,
+typedef void (*convolve_fn_t)(const uint8_t *src, ptrdiff_t src_stride,
+                              uint8_t *dst, ptrdiff_t dst_stride,
                               const int16_t *filter_x, int filter_x_stride,
                               const int16_t *filter_y, int filter_y_stride,
                               int w, int h);