The new renderers don't support them due to the required complexity of
integrating them with Vulkan and the assumptions those nodes make about
the renderer (the GL renderer exports its internal APIs into the
GLShader).
There haven't been any complaints that I'm aware of since 4.14 was
released where the default renderer does not support the nodes, so usage
in public seems to be close to nonexistant.
The 2 uses I know of were workarounds about missing features in GTK that
have stopped since GTK now supports them:
1. GStreamer used in to do premultiplication when the old GL renderer
did not do so in hardware but on the CPU.
2. Adwaita used it for masking before the mask node wa added in 4.10.
These tests come in two variants.
The first takes .node and .offload file, parses the node file,
and compares the resulting subsurface attachments to expected results.
The second variant takes two .node/.offload file pairs and a .diff
file, parses the node files, compares the resulting subsurface
attachments, and then diffs the nodes, comparing the resulting
area to the region in the .diff file.
