rBe0c088f8fb5a introduced offline rendering support on windows, sadly it was trying to use the desktop window for getting a context, which given SetPixelFormat can only be called once for any given HDC was an unfortunate choice.
This patch uses a temporary hidden window for getting the opengl context.
Reviewers: mano-wii
Differential Revision: https://developer.blender.org/D3481
8c77c36539