Use git clone --recurse-submodules -b 1.1.0 https://github.com/mltframework/webvfx to make building with qmelt easier.
This version adds support for MLT v6.20.0's preview scaling.
For the MLT integration, if the resource starts with "plain:" webvfx does automatic scaling per the zoom level. This works fine for content that is simply produced or overlayed by the filter. However, content that uses the webvfx JavaScript extension will not operate with zoom unless the service property mlt_resolution_scale is set to 1. New properties mlt_profile_scale_width and mlt_profile_scale_height as real numbers that reflect the new MLT consumer scale property used to do low-res preview scaling. Content script can access this via webvfx.getNumberParameter() to scale its parameters and elements.