[mutter] screen-cast: Add 'dma-buf-screen-sharing' experimental feature

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit af9df1e5b62b253e5f1d5f6eff89e45e3bed81b3
Author: Jonas Ådahl <jadahl gmail com>
Date:   Mon Sep 14 18:23:10 2020 +0200

    screen-cast: Add 'dma-buf-screen-sharing' experimental feature
    
    It's enabled by default when using the i915 driver, but disabled
    everywhere else until it can be made reliably an improvement. Until
    then, for anyone want to force-enable it, add the string
    'dma-buf-screen-sharing' to the experimental features list in GSettings.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1442

 src/backends/meta-settings-private.h      | 1 +
 src/backends/meta-settings.c              | 2 ++
 src/backends/native/meta-backend-native.c | 5 +++++
 3 files changed, 8 insertions(+)
---
diff --git a/src/backends/meta-settings-private.h b/src/backends/meta-settings-private.h
index 91b3eb17a7..c0621cb080 100644
--- a/src/backends/meta-settings-private.h
+++ b/src/backends/meta-settings-private.h
@@ -35,6 +35,7 @@ typedef enum _MetaExperimentalFeature
   META_EXPERIMENTAL_FEATURE_KMS_MODIFIERS  = (1 << 1),
   META_EXPERIMENTAL_FEATURE_RT_SCHEDULER = (1 << 2),
   META_EXPERIMENTAL_FEATURE_AUTOSTART_XWAYLAND  = (1 << 3),
+  META_EXPERIMENTAL_FEATURE_DMA_BUF_SCREEN_SHARING = (1 << 4),
 } MetaExperimentalFeature;
 
 typedef enum _MetaXwaylandExtension
diff --git a/src/backends/meta-settings.c b/src/backends/meta-settings.c
index f14dbfae61..553e2cd1c8 100644
--- a/src/backends/meta-settings.c
+++ b/src/backends/meta-settings.c
@@ -273,6 +273,8 @@ experimental_features_handler (GVariant *features_variant,
         feature = META_EXPERIMENTAL_FEATURE_RT_SCHEDULER;
       else if (g_str_equal (feature_str, "autostart-xwayland"))
         feature = META_EXPERIMENTAL_FEATURE_AUTOSTART_XWAYLAND;
+      else if (g_str_equal (feature_str, "dma-buf-screen-sharing"))
+        feature = META_EXPERIMENTAL_FEATURE_DMA_BUF_SCREEN_SHARING;
 
       if (feature)
         g_message ("Enabling experimental feature '%s'", feature_str);
diff --git a/src/backends/native/meta-backend-native.c b/src/backends/native/meta-backend-native.c
index 1b7c8e0581..73723ace92 100644
--- a/src/backends/native/meta-backend-native.c
+++ b/src/backends/native/meta-backend-native.c
@@ -347,6 +347,7 @@ static void
 maybe_disable_screen_cast_dma_bufs (MetaBackendNative *native)
 {
   MetaBackend *backend = META_BACKEND (native);
+  MetaSettings *settings = meta_backend_get_settings (backend);
   MetaRenderer *renderer = meta_backend_get_renderer (backend);
   MetaRendererNative *renderer_native = META_RENDERER_NATIVE (renderer);
   MetaScreenCast *screen_cast = meta_backend_get_screen_cast (backend);
@@ -365,6 +366,10 @@ maybe_disable_screen_cast_dma_bufs (MetaBackendNative *native)
   if (g_strv_contains (enable_dma_buf_drivers, driver_name))
     return;
 
+  if (meta_settings_is_experimental_feature_enabled (settings,
+        META_EXPERIMENTAL_FEATURE_DMA_BUF_SCREEN_SHARING))
+    return;
+
   g_message ("Disabling DMA buffer screen sharing for driver '%s'.",
              driver_name);