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.
Since the corresponding source files do not include "config.h", and are
not compiled with `GTK_COMPILATION`, they will generate the wrong symbol
exporting under Windows.
Add adds a demo showing off GskGLShaderNode in various ways.
It has a transistion widget, using some examples from
gl-transitions.com, with child widgets being both images, a GL area
and real widgets (that let you edit the transition shaders
themselves.
It also has a fancy fire effect on hove on the buttons.
