darktable: fix OpenCL support

The OpenCL compiler does not properly support -I flags to include the
kernel paths, breaking relative includes. Simply replace the included
files by absolute paths.

OpenCL kernels verified to compile and work on an RX 580.

Credits for the fix go to nixos-rocm.

pkgs/applications/graphics/darktable/default.nix

@@ -43,6 +43,10 @@ stdenv.mkDerivation rec {
     libPathEnvVar = if stdenv.isDarwin then "DYLD_LIBRARY_PATH" else "LD_LIBRARY_PATH";
     libPathPrefix = "$out/lib/darktable" + stdenv.lib.optionalString stdenv.isLinux ":${ocl-icd}/lib";
   in ''
+    for f in $out/share/darktable/kernels/*.cl; do
+      sed -r "s|#include \"(.*)\"|#include \"$out/share/darktable/kernels/\1\"|g" -i "$f"
+    done
+
     gappsWrapperArgs+=(
       --prefix ${libPathEnvVar} ":" "${libPathPrefix}"
     )