I've tried just about every single TileMill compositing operation on the vector and raster layers in question, and I have to admit that I can't figure it out: How the heck can I mask a raster with a vector in TileMill/CartoCSS?
Basically, I have a raster, generated by QGIS (and masked in QGIS with a vector shapefile of the U.S.). I ran it through GDAL to colorize it and re-project it into web mercator for TileMill. Somewhere in the process, everything outside the QGIS mask became a solid white pixel, and I just can't seem to trim that white off. I've brought the same shapefile into TileMill, and that's what I'm trying to use to trim the raster.
As usual, I feel like there's something basic I didn't do—maybe in the QGIS GeoTIFF export process?—that would explain TileMill's stubborn refusal to do away with the gross white square around my map, but Google is failing me. Any ideas?