Declare the 'src' parameters as const.

The 'src' parameter of aom_img_downshift() was changed to const in
commit bf9d6d4b3a7e2496aaf226784861882bcba72322. This CL makes the
same change to the 'src' parameters of aom_img_upshift() and
aom_img_truncate_16_to_8() for consistency. It also declares the
p_src local variables as const.

Change-Id: I8b0b4895657891cbcad7b4c6edf2839444e5e196
diff --git a/common/tools_common.h b/common/tools_common.h
index 4811de4..5879036 100644
--- a/common/tools_common.h
+++ b/common/tools_common.h
@@ -152,10 +152,10 @@
 int aom_img_read(aom_image_t *img, FILE *file);
 
 double sse_to_psnr(double samples, double peak, double mse);
-void aom_img_upshift(aom_image_t *dst, aom_image_t *src, int input_shift);
+void aom_img_upshift(aom_image_t *dst, const aom_image_t *src, int input_shift);
 void aom_img_downshift(aom_image_t *dst, const aom_image_t *src,
                        int down_shift);
-void aom_img_truncate_16_to_8(aom_image_t *dst, aom_image_t *src);
+void aom_img_truncate_16_to_8(aom_image_t *dst, const aom_image_t *src);
 
 #ifdef __cplusplus
 } /* extern "C" */