These don't necessarily go in any group but are required for dispatch to
work properly. The trampoline is a compute shader that is the initial
start point for the trace. It's in charge of invoking the actual
ray-gen shader. The trivial return shader is used whenever another
shader is missing and it does no work except the minimum required to do
a stack return.
v2: Rebase on upstream changes (Lionel)
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8637>
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