These positions are not guaranteed to be in a specific order when linked
into the final GPU program. They need to be specified so that our code
in gskglrenderer.c can use known positions for them to match up with
our GskQuadVertex.
This fixes the GL renderer on macOS's OpenGL shader compiler.
Fixes#3420
For vulkan/broadway this just means to ignore it, but for the gl
backend we support (with up to 4 texture inputs, which is similar to
what shadertoy does, so should be widely supported).
Print out the full assembled shader sources when
GSK_DEBUG=shaders is given. This is very verbose,
but may be useful to see what we actually pass
to the compiler.
Almost always the source is created by combining various sources, which
means the line numbers in the error messages are hard to use. Adding
the line numbers to the source in the error message helps with this.
