Add OTS_TAG()

[behdad]

Currently OTS doesn't provide any macro to create an integer table tag to pass to getTableAction(). Would be nice to copy HB_TAG() so it's readily available. See eg:

https://codereview.chromium.org/954953002

[khaledhosny]

It should be trivial, but we had a similar macro that was removed in e14e0fa and I wonder what big/little endian issues OTS has that HarfBuzz doesn’t?

[behdad]

Ok so first we have to figure out whether the tag OTS expects is endian-dependent or not. We might need to fix that first.

[behdad]

Ok, I studied that all. It's safe to add the tag macro again. Internally OTS keeps tags in endian-specific order, but this function always accepts them big-endian, as can be seen in ots.cc, there's a htonl() before calling the virtual function:

ots::TableAction GetTableAction(ots::OpenTypeFile *header, uint32_t tag) { 
  ots::TableAction action = ots::TABLE_ACTION_DEFAULT; 

  action = header->context->GetTableAction(htonl(tag));