Ensure fills with transparent colors and patterns are rendered as expected

[lucaswoj]

This PR fixes #3107 by making FillStyleLayer#isHidden aware of fill-pattern.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality Add regression test for mapbox-gl-js#3107 mapbox-gl-test-suite#140
• manually test the debug page
• get a PR review

[lucaswoj]

cc @dpieri @jfirebaugh @mourner @mollymerp

[jfirebaugh]

LGTM, although I question whether isHidden should be checking any properties other than visibility.

[lucaswoj]

LGTM, although I question whether isHidden should be checking any properties other than visibility.

That's a fair point. I'd be happy to refactor to make it so. Should we go for it?

[jfirebaugh]

If it doesn't regress tests or benchmarks, I say go for it.

[lucaswoj]

The latest commit removes all opacity tests from the StyleLayer#isHidden method. This is unlikely to cause performance regressions in core styles because we don't often have 0 opacity layers in core styles. It's possible a 3rd party style may see a perf regression due to this change.