Version 2.21.0

Changed

• Switched to C++14 as minimum requirement
• Removed suport for Google Sparsehash
• Removed support for projection using the Proj library (which only worked for very old versions of Proj)
• Various refactoring and code cleanups

Fixed

• Fix problem where bz2 files were not read completely (#373)
• Order deleted objects after visible ones in reverse id order. This fixes a problem when merging diffs for different extracts.
• Remove resource leak in temp file creation