build: Set proper defaults for media backends

GTK supports webm playback, which means a backend should always be
compiled.

The ffmpeg backend however is incomplete (no audio) and as such, we
don't want people to end up with it accidentally.

Since we don't want to drag an entire gstreamer build into our ci
on MacOs or msvc, explicitly disable the gstreame media backend there.

.gitlab-ci.yml

@@ -172,6 +172,7 @@ macos:
     - meson -Dx11-backend=false
             -Dbroadway-backend=true
             -Dmacos-backend=true
+            -Dmedia-gstreamer=disabled
             -Dintrospection=disabled
             -Dcpp_std=c++11
             -Dpixman:tests=disabled

.gitlab-ci/test-msvc.bat

@@ -6,7 +6,7 @@ call "C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\VC\Auxiliar
 
 :: FIXME: make warnings fatal
 pip3 install --upgrade --user meson==0.56.2  || goto :error
-meson _build || goto :error
+meson -Dmedia-gstreamer=disabled _build || goto :error
 ninja -C _build || goto :error
 
 goto :EOF

meson_options.txt

@@ -26,15 +26,17 @@ option('macos-backend',
        description : 'Enable the macOS gdk backend (only when building on macOS)')
 
 # Media backends
+# For distros: GTK guarantees support for WebM video (VP8 and VP9), so a supported build
+# should provide that.
 
 option('media-ffmpeg',
        type: 'feature',
-       value: 'auto',
-       description : 'Build the ffmpeg media backend')
+       value: 'disabled',
+       description : 'Build the experimental ffmpeg media backend')
 
 option('media-gstreamer',
        type: 'feature',
-       value: 'auto',
+       value: 'enabled',
        description : 'Build the gstreamer media backend')
 
 # Print backends