--merge with directory output

[kleunen]

When I use --merge with directory output (not sqlite), and the directory already exists, I get an exception:

terminate called after throwing an instance of 'std::runtime_error' what(): out of memory Aborted (core dumped)

[systemed]

Hm, yes, --merge isn't really supported for directory output (we don't have any code to read tiles from file, for starters).

[kleunen]

Yes, i think it "crashes" here: https://github.com/systemed/tilemaker/blob/master/src/tilemaker.cpp#L439-L443

Maybe add sqlite in the if, and ignore --merge when not sqlite. Or give an error message on startup when --merge is used with directory output. Because i was reading the whole osm.pbf file (for 30 minutes), and then get the exception. Which was very unclear.