dxvk: support version-specific MoltenVK patches

While it’s unlikely, it’s possible that different MoltenVK versions
could require their own compatability patches. Support that by making
the `moltenvk` derivation provide the patch via `passthru`. There is no
package with the patch applied because the patch should never be used by
anything other than DXVK.

pkgs/misc/dxvk/default.nix

@@ -72,6 +72,7 @@ let
 
     # DXVK with MoltenVK requires a patched MoltenVK in addition to its own patches. Provide a
     # convenience function to handle the necessary patching.
+    #
     # Usage:
     # let
     #   patchedMoltenVK = dxvk.patchMoltenVK darwin.moltenvk;
@@ -80,9 +81,10 @@ let
     passthru.patchMoltenVK = moltenvk:
       moltenvk.overrideAttrs (old: {
         patches = old.patches or [ ] ++ [
-          # Lie to DXVK about certain features that DXVK expects to be available and set defaults
-          # for better performance/compatability on certain hardware.
-          ./darwin-moltenvk-compat.patch
+          # Apply MoltenVK’s DXVK compatability patch. This is needed to fake support for certain
+          # extensions. There is no package for a patched MoltenVK to avoid any confusion by users
+          # whether they should use it. Except with DXVK, the answer is always no.
+          old.passthru.dxvkPatch
         ];
       });
 

pkgs/os-specific/darwin/moltenvk/default.nix

@@ -43,6 +43,10 @@ stdenvNoCC.mkDerivation rec {
   # MoltenVK requires specific versions of its dependencies.
   # Pin them here except for cereal, which is four years old and has several CVEs.
   passthru = {
+    # The patch required to support DXVK may different from version to version. This should never
+    # be used except with DXVK, so there’s no package for it. To emphasize that this patch should
+    # never be used except with DXVK, `dxvk` provides a function for applying this patch.
+    dxvkPatch = ./dxvk-moltenvk-compat.patch;
     glslang = (glslang.overrideAttrs (old: {
       src = fetchFromGitHub {
         owner = "KhronosGroup";