Add PropertiesID field to Metadata structs

[flibitijibibo]

Not much to see here, except one detail: The rest of the metadata struct does not have to be initialized since it's mostly an out struct, but the only way Properties will work here is if that one field is initialized first. The idea is that shadercross would check props for known metadata extensions, then fill in the properties found when applicable.

Kind of weird but it does work - if we still don't want to do this we can just call this WONTFIX.

Fixes #72

[thatcosmonaut]

Maybe we should note on the out structs that the client has to free the properties if they are not 0.

[flibitijibibo]

Tweaked the docs accordingly.