This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
On the only platform where PVRTC is supported (iOS),
ETC2 generally supersedes PVRTC in every possible way. The increased
memory usage is not really a problem thanks to modern iOS' devices
processing power being higher than its Android counterparts.
