Consume official GStreamer binaries on MacOS

This PR re-enables support for the gstreamer mediastack
in macOS by consuming the official binary '.pkg' files
from gstreamer.freedesktop.org

To maintain symmetry with other platforms, the '.pkg'
files are uploaded to servo-build-deps and fetched from
there using the new script 'etc/install_macos_gstreamer.sh'.

Unlike the Homebrew version, the official GStreamer is
distributed as a 'relocatable' framework i.e the dylibs all
have @rpath-relative install names and also link to other
dylibs using @rpath relative path. To address this difference
the 'servo' binary needs to be patched with 'install_name_tool'
to add an LC_RPATH command that sets the relative paths
that the dynamic linker should search when trying to satify
dependencies. In Servo's case, this will be a path relative to
the 'servo' binary itself i.e '@executable_path/lib/'

The additional 'lib' is due to a flaw in the gstreamer
packaging where the install names of some of the dylibs
have the prefix '@rpath/lib' and some of them just have '@rpath'.

This PR also fixes a couple of issues present in the
`mach build` process on MacOS:
1. `mach build` process was not copying transitive dependencies
   of servo binary but only the first level dylibs
2. `mach build` process didn't patch the links to dylibs
   in servo binary (and dependencies). This meant though
   (some) dylibs were copied to local path, the binary
   still loaded the dylibs from system GStreamer installation
   i.e homebrew instead of the copieds dylibs

The build and runtime dependencies in etc/homebrew/Brewfile
and etc/homebrew/Brewfile-build have also been removed in This
PR.

Signed-off-by: Mukilan Thiyagarajan <me@mukilan.in>

.github/workflows/mac.yml

@@ -66,18 +66,11 @@ jobs:
       - name: Bootstrap
         run: |
           python3 -m pip install --upgrade pip virtualenv
-          brew bundle install --verbose --no-upgrade --file=etc/homebrew/Brewfile
-          brew bundle install --verbose --no-upgrade --file=etc/homebrew/Brewfile-build
-          rm -rf /usr/local/etc/openssl
-          rm -rf /usr/local/etc/openssl@1.1
-          brew install openssl@1.1 gnu-tar
+          bash etc/install_macos_gstreamer.sh
+          brew install gnu-tar
       - name: Release build
         run: |
-          export OPENSSL_INCLUDE_DIR="$(brew --prefix openssl)/include"
-          export OPENSSL_LIB_DIR="$(brew --prefix openssl)/lib"
-          export PKG_CONFIG_PATH="$(brew --prefix libffi)/lib/pkgconfig/"
-          export PKG_CONFIG_PATH="$(brew --prefix zlib)/lib/pkgconfig/:$PKG_CONFIG_PATH"
-          python3 ./mach build --release --media-stack=dummy --with-${{ env.LAYOUT }}
+          python3 ./mach build --release --with-${{ env.LAYOUT }}
       - name: Smoketest
         run: python3 ./mach smoketest
       - name: Unit tests