internal/rendertest: lower test tolerances

Egon Elbre pointed out that a difference of 20 means a 10% difference.
Lowering the tolerance to 5 didn't work on my setup; leave it at 10.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/rendertest/util_test.go

@@ -176,7 +176,7 @@ func close(b1, b2 uint8) bool {
 		b1, b2 = b2, b1
 	}
 	diff := b2 - b1
-	return diff < 20
+	return diff < 10
 }
 
 func (r result) expect(x, y int, col color.RGBA) {