Touching color consistency with Scratch 2

[paulkaplan]

This is an issue to track example projects and repros of inconsistencies with touching-color between scratch 3 and scratch 2. This was inspired by scratchfoundation/scratch-gui#689, which gives the first example case:

• https://llk.github.io/scratch-gui/develop/#171494108

[thisandagain]

Here is a more minimal repro that demonstrates the issue. While attempting to repo I noticed that choosing a darker color for "<touching color [ ]>" is more likely to result in a difference in behavior between 2.0 and 3.0. You can test this in the repro project below by choosing a different "red" from the backdrop.

https://scratch.mit.edu/projects/193005590/
https://llk.github.io/scratch-gui/develop/#193005590

[paulkaplan]

This PR (and comments) provide some pretty good background, @cwillisf has put some good thinking into this issue already #162