Don't use WebGL if there is a major performance caveat
Improve type checking in WebGL support test
Merge pull request #1330 from twpayne/fail-if-major-performance-caveat

Don't use WebGL if there as a major performance caveat
Add ol.structs.RBush#isEmpty
Merge pull request #1331 from twpayne/rbush-is-empty

Add ol.structs.RBush#isEmpty
Merge remote-tracking branch 'upstream/master'