The new version has an API change. Also, up our SDK requirements to be a
bit more realistic.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

And fix a zillion omissions this uncovered.

We can just use Canvas now.  The two are the same thing.

This makes skyanalyzer also check the examples, and fixes everything it
found there.

Teleportation for physics bodies works again

Refactors the sprite matrix handling to do most calculations in Dart, minimizes the number of calls down to Skia