Potentialy faster array allocation in rbush
Faster vector source clear

Three seconds speed up for clearing 100'000 features.
Introduce clear event on vector source

Three seconds speed up when clearing 100'000 features.
Clearing is now around 350ms.
Add optional fast parameter for clearing vector source
Merge pull request #3013 from gberaudo/performance_vector_source_clear

Improve vector source clear() performance