General build fixes for MLT plugin and qmelt on Windows.
Shotcut-specific static build of libwebvfx on Windows.
Fix build regression on Linux.
Fix deadlock calling mlt_consumer_stop() from Qt.

mlt_consumer_stop() is trying to join the render thread, but the render
thread was blocked on WebVfx::render() occurring on main Qt thread.
Adding WebVfx::renderComplete() lets us finish rendering normally as
well as cancel the wait-for-render inside of a new MLT consumer-stopping
event listener.
Merge branch 'master' of https://github.com/ddennedy/webvfx

Conflicts:
	mlt/service_manager.cpp
	mlt/service_manager.h
	webvfx/effects_impl.cpp