scenery fill/stroke doesn't support TinyProperty

[zepumph]

I was surprised to see many spots with instanceof ReadOnlyProperty without any notion of TinyProperty. I'll take a look. I got bit for ~2 hours on this over in phetsims/buoyancy#96 thinking I had done something wrong in THREE.js when actually it was just that I was using a TinyProperty<Color> instead of a Property<Color>.

[zepumph]

For review:

• I ended up looking for all usages of instanceof ReadOnlyProperty || instanceof TinyProperty. So it leaked a bit outside of scenery
• In Paintable, I noticed had to add Paint as a return type for fill and stroke. Do we need the specific types here too? Right now it is like LinearGradient | RadialGradient | Pattern | Paint
• I'm really happy about how typesafe this makes a lot of our code. Note the couple of type cast as calls I could get rid of.
• Please note how I created a Type parameter for isTReadOnlyProperty so that we could pass through the non Property type that we wanted to pull out of the return type. It is hard coded, but I think it is better than nothing (needed for Paintable changes with the return type).
• I did end up finding some pixel comparison changes, but they were very very slight from this change, and so I still decided to commit. Here is an example of what changed in BASE and GE:E

@jonathanolson can you please review the above commits? Thanks.

[zepumph]

This issue mainly started because I thought that b8f7e12 would be a simple and easy change, but it actually opened up can of worms. I'm glad that I hit this instead of someone else since it seems so obvious that TReadOnlyProperty should behave the same when passing into scenery options.

[zepumph]

I'll test snapshot again on firefox, because that may be chromes fault (says @jonathanolson).

[zepumph]

The pixel comparisons on firefox showed no changes. Thanks for the tip @jonathanolson.