CI: Add PipeWire 0.3.40 to Flatpak

We require PipeWire 0.3.40 or later in preparation for proper
DMA-BUF handling, which includes negotiation of modifier lists and
dropping of single modifiers.

This commit should be reverted as soon as the Freedesktop Flatpak SDK
contains this or a newer PipeWire version.

CI/flatpak/com.obsproject.Studio.json

@@ -237,6 +237,32 @@
         }
       ]
     },
+    {
+      "name": "pipewire",
+      "buildsystem": "meson",
+      "config-opts": [
+        "-Daudiotestsrc=disabled",
+        "-Droc=disabled",
+        "-Dvideotestsrc=disabled",
+        "-Dvolume=disabled",
+        "-Dvulkan=disabled",
+        "-Ddocs=disabled",
+        "-Dman=disabled",
+        "-Dbluez5-codec-ldac=disabled",
+        "-Dbluez5-codec-aptx=disabled",
+        "-Dlibcamera=disabled",
+        "-Dudevrulesdir=/app/lib/udev/rules.d/",
+        "-Dsession-managers=[]"
+      ],
+      "sources": [
+        {
+          "type": "git",
+          "url": "https://gitlab.freedesktop.org/pipewire/pipewire.git",
+          "tag": "0.3.40",
+          "commit": "7afd80052b7c49754a13c9ab49c368f95b60e0a7"
+        }
+      ]
+    },
     {
       "name": "cef",
       "buildsystem": "cmake-ninja",