Add missing `ID2D1Multithread`

A complete runtime environment for gcc

Brought to you by: jon_y, ktietz70, nightstrike

Milestone: Next Release (example)

Status: open

Owner: nobody

Labels: Headers (19) directx (2) direct2d (1)

Priority: 5

Updated: 2017-07-25

Created: 2016-12-24

Private: No

The Microsoft Windows SDK declares interface ID2D1Multithread in file 'd2d1_1h'. But there is no such interface declared in mingw-w64's file.

Please, add this interface to mingw-w64 headers.