qmlglsink: Fix HDC for wglMakeCurrent() calls - enables multiple instances on Windows
This patch aims to fix issue discussed at #477
There is one known issue - I did not update meson.build related to ext/qt. I'd kindly ask someone who has experience with meson and ability to test it on Windows to port my changes from qtplugin.pro into meson.build (just add user32.lib to link options).