tests: constify fill_image_with_color()

Const has documentary value saying the code will not modify the
parameter contents. Everything that can be const, should be const.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

tests/weston-test-client-helper.c

@@ -1867,7 +1867,7 @@ client_create_viewport(struct client *client)
  * \param color The color to use.
  */
 void
-fill_image_with_color(pixman_image_t *image, pixman_color_t *color)
+fill_image_with_color(pixman_image_t *image, const pixman_color_t *color)
 {
 	pixman_image_t *solid;
 	int width;

tests/weston-test-client-helper.h

@@ -280,7 +280,7 @@ struct wp_viewport *
 client_create_viewport(struct client *client);
 
 void
-fill_image_with_color(pixman_image_t *image, pixman_color_t *color);
+fill_image_with_color(pixman_image_t *image, const pixman_color_t *color);
 
 pixman_color_t *
 color_rgb888(pixman_color_t *tmp, uint8_t r, uint8_t g, uint8_t b);