ci: fix the Windows, Linux, macOS and ARM64 failures

- Windows: vcpkg ships `pkgconf` without the `pkg-config` shim, so the
  oak-ffmpeg-link build script failed with "program not found". Probe
  `pkg-config`, fall back to `pkgconf` (or honor `PKG_CONFIG`), and join
  the child's `PKG_CONFIG_PATH` with the platform separator instead of a
  hard-coded `:` (which split `C:\...` apart).
- Linux (and openKylin): the test binaries link the VAAPI stack via
  vcpkg's FFmpeg; install the `libva2`/`libva-drm2`/`libvdpau1` runtime
  packages the loader needs.
- macOS: bump the gpui submodule
  (OakVideoEditorCommunity/oak-gpui@fix/macos-metal-layer-and-dead-code):
  `setColorspace:` now sends to the `MetalLayerRef` (`self.layer.as_ref()`
  made `&*layer` the owned type, which is not `objc::Message`), and the
  viewer's `GpuFrameEntry` carries the non-Linux dead-code allowance.
- openKylin ARM64: switch the ocio patch to the fork's
  fix/aarch64-c-char rev. Upstream models C `char*` as `*const i8`;
  aarch64's `c_char` is u8, so the crates did not compile. The fix uses
  `c_char` throughout ocio-sys and the ocio-rs boundary (pushed as
  30338c6a169bbbada862fb3ac256e79b656159cf).

.github/workflows/ci.yml

@@ -50,6 +50,7 @@ jobs:
             libasound2-dev libpulse-dev libsndfile1-dev \
             libgl1-mesa-dev libgl1-mesa-dri mesa-vulkan-drivers \
             libvulkan-dev libxkbcommon-dev libxkbcommon-x11-dev xvfb libdrm-dev \
+            libva2 libva-drm2 libvdpau1 \
             autoconf autoconf-archive automake libtool
 
       # ------------------------------------------------------------------
@@ -463,6 +464,7 @@ jobs:
             libasound2-dev libpulse-dev libsndfile1-dev \
             libgl1-mesa-dev libgl1-mesa-dri mesa-vulkan-drivers \
             libvulkan-dev libxkbcommon-dev libxkbcommon-x11-dev xvfb libdrm-dev \
+            libva2 libva-drm2 libvdpau1 \
             gdb file autoconf autoconf-archive automake libtool
 
       # ------------------------------------------------------------------