This fixes the vendor ICD load path to work with NixOS's opengl path.
Previously, the only workaround to load ICD files was to manually
override the load path at runtime using an environment variable.
Tested against Intel and Nvidia drivers using the clinfo package:
Before:
```
$ clinfo
Number of platforms 0
```
After:
```
$ clinfo
Number of platforms 1
Platform Name NVIDIA CUDA
Platform Vendor NVIDIA Corporation
<...>
```
For APIs that use driver config files, the config files should point to libraries using absolute paths, so `RUNPATH` should be unnecessary. It is still needed for `libGLX` because that does not use config files but the dispatch library receives just a vendor name from the X server.
Previously we were relying on LD_LIBRARY_PATH to discover driver libraries (libGL, ligGLX, libEGL, OpenCL and Vulkan). This has the problem that setuid programs (in particular VirtualBox) ignore LD_LIBRARY_PATH. Fix it by setting RUNPATH in various dispatch libraries.
This is not needed for libvdpau because it is already configured to look for libraries in the driver paths.
Fixes https://github.com/NixOS/nixpkgs/issues/22760.
* Implement libGL as a symlink package which uses libraries from libglvnd and
headers from Mesa (since ones from libglvnd are outdated).
* Use libGL_driver.driverLink treewide; add FHS paths where possible.
I updated the erlang esdl lib, added the erlang cl lib,
added opencl-headers and ocl-icd to make wings build and run.
I have not tested its opencl part; I only added dependencies so
it builds.
