Blender Git Loki

install_deps.sh: No longer forcing Alembic sources to be redownloaded

For no apparent reason, when building Alembic the script would always
re-download and re-extract the Alembic source code. This is no longer the
case, and it now only happens if the source directory is missing. Since the
source directory name contains the Alembic version, it will automatically
trigger a download+extract when the version changes.