Prevent Qt from using system fonts

[prismatic]

I'm trying to get Qt to use only font files I specify, but it reverts to loading fonts from my system instead. For example, I call something like:

@FontLoader { id: sourcesanspro; source: "fonts/SourceSansPro-Semibold.ttf"; }@

Then use the font in a Text item by specifying
@font.family: sourcesanspro.name;@

But Qt looks up the same font from my system instead. It won't use the "Semibold" variant, and instead load whatever it thinks the "regular" weight of the font is. I don't want this behavior -- I just want it to use the specified file and to ignore all other fonts. How can I do this?