tests/alpha-blending: use two_norm tolerance

Switch from per-channel max error tolerance to max two-norm (Euclidean
distance) error. Geometrically this means that previously the accepted
volume was a +/- tolerance cube around the reference point, and now it
is a sphere with tolerance radius. This makes the check slightly
stricter.

The real benefit is simplifying the code.

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

tests/alpha-blending-test.c

@@ -229,7 +229,6 @@ check_blend_pattern(struct buffer *bg, struct buffer *fg, struct buffer *shot,
 	struct rgb_diff_stat diffstat = { .dump = dump, };
 	bool ret = true;
 	int x;
-	unsigned i;
 
 	for (x = 0; x < BLOCK_WIDTH * ALPHA_STEPS - 1; x++) {
 		if (!pixels_monotonic(shot_row, x))
@@ -239,11 +238,8 @@ check_blend_pattern(struct buffer *bg, struct buffer *fg, struct buffer *shot,
 					    &diffstat, space);
 	}
 
-	for (i = 0; i < COLOR_CHAN_NUM; i++) {
-		if (diffstat.rgb[i].min <= -tolerance ||
-		    diffstat.rgb[i].max >= tolerance)
+	if (diffstat.two_norm.max > tolerance)
 		ret = false;
-	}
 
 	rgb_diff_stat_print(&diffstat, __func__, 8);