lib/mingle: [nfc] pre-emptively squash -fpermissive warning

lib/mingle: [nfc] pre-emptively squash -fpermissive warning

When moving this code to C++ in an upcoming commit, this would otherwise
trigger:

  lib/mingle/ink.cpp:46:22: error: uninitialized const 'Origin' [-fpermissive]
   static const point_t Origin;
                        ^
  In file included from lib/mingle/ink.cpp:17:0:
  lib/mingle/ink.h:19:16: note: 'const struct point_t' has no user-provided
    default constructor
   typedef struct {
                  ^
  lib/mingle/ink.h:20:10: note: and the implicitly-defined constructor does not
    initialize 'double point_t::x'
     double x, y;
          ^

This noisy initialization can probably be undone after mingle and libmingle are
moved to entirely C++.

Gitlab: #2154