Make "touching color (white)" match Scratch

[adroitwhiz]

When I wrote the WebGL renderer, the behavior of "touching color (white)" when a sprite is over a transparent part of the stage's backdrop was not specified in Scratch itself. Because the color that appears under transparent parts of the backdrop is white, it would seem that "touching color (white)" would return true in such a case, but that wasn't how Scratch 2.0 behaved, and it was only in https://github.com/LLK/scratch-render/pull/489 that it was decided that 3.0 would break compatibility with 2.0 and return true.

Now that the behavior is specified in 3.0, Leopard should match it.