Revision 543591

Update to upstream prerelease snapshot 1.0.29 as of 20200620

Details:
* Pull source code from GitHub
* Switch to cmake
* Remove clipping option (autodetected)
* Fix denial-of-service (CVE-2019-3832, since CVE-2018-19758 appears
  to be incomplete.)

PR:		248268
Submitted by:	daniel.engberg.lists@pyret.net
MFH:		2020Q3 (hat: ports-secteam)
Security:	CVE-2019-3832