community/mesa-demos: Install necessary shader files

This fixes the demos which require shader files. For example,

$ mandelbrot
Unable to open shader file CH18-mandel.vert
Unable to open shader file CH18-mandel.frag
Assertion failed: vertShader || fragShader (../src/util/shaderutil.c: LinkShaders3: 116)
Aborted (core dumped)

The issue was first reported in the following bug report.
https://bugzilla.redhat.com/show_bug.cgi?id=1373299

It was fixed in the following commit of Fedora 39.
https://src.fedoraproject.org/rpms/mesa-demos/c/30fa49f5fdff5b91125672f3bbc1eef30b2251d8?branch=f39

assigned to @ncopa

added 245 commits

• 31b4e677...b281f37a - 244 commits from branch alpine:master
• e4f6215f - community/mesa-demos: Install necessary shader files

Compare with previous version

Is there any upstream bug-report?

merged

The patch was added in this upstream merge request https://gitlab.freedesktop.org/mesa/demos/-/merge_requests/153

I did not find the related issue but the patch file in that merge request is same as the Fedora one.