tests: expand allowed pixel fuzz for GL

Running with Mesa 20.1.0-devel (git-c7617d8908) GL renderer:
Radeon RX 550 Series (POLARIS11, DRM 3.27.0, 4.19.0-2-amd64, LLVM 8.0.1)

I found output-tranform test (a future patch) to produce exactly this much more
difference between Pixman and GL rendererers.

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

tests/weston-test-client-helper.c

@@ -1655,7 +1655,7 @@ verify_screen_content(struct client *client,
 		      int seq_no)
 {
 	const char *test_name = get_test_name();
-	const struct range gl_fuzz = { 0, 1 };
+	const struct range gl_fuzz = { -3, 4 };
 	struct buffer *shot;
 	pixman_image_t *ref = NULL;
 	char *ref_fname = NULL;