app/headless,internal/rendertest: relax tests

The Mesa software OpenGL implementation strays enough from the
reference values that tests fail. Relax the tests to make them
pass again.

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

internal/rendertest/util_test.go

@@ -188,7 +188,7 @@ func close(b1, b2 uint8) bool {
 		b1, b2 = b2, b1
 	}
 	diff := b2 - b1
-	return diff < 10
+	return diff < 16
 }
 
 func (r result) expect(x, y int, col color.RGBA) {