libmutter15: use hex colors with Cogl.Color.from_string

`Cogl.Color.from_string` can't accept color names like
`Clutter.Color.from_string` which results in a segfault with mutter47+

Author: garaevdi

src/Background/Background.vala

@@ -103,7 +103,7 @@ public class Gala.Background : Object {
 #endif
         if (color == null) {
 #if HAS_MUTTER47
-            color = Cogl.Color.from_string ("black");
+            color = Cogl.Color.from_string ("#000000");
 #else
             color = Clutter.Color.from_string ("black");
 #endif
@@ -122,7 +122,7 @@ public class Gala.Background : Object {
 #endif
             if (second_color == null) {
 #if HAS_MUTTER47
-                second_color = Cogl.Color.from_string ("black");
+                second_color = Cogl.Color.from_string ("#000000");
 #else
                 second_color = Clutter.Color.from_string ("black");
 #endif

src/Background/BackgroundContainer.vala

@@ -29,7 +29,7 @@ public class Gala.BackgroundContainer : Meta.BackgroundGroup {
         });
 
 #if HAS_MUTTER47
-        background_color = Cogl.Color.from_string ("Black");
+        background_color = Cogl.Color.from_string ("#000000");
 #else
         background_color = Clutter.Color.from_string ("Black");
 #endif

src/ScreenshotManager.vala

@@ -244,7 +244,7 @@ public class Gala.ScreenshotManager : Object {
         flash_actor.set_size (width, height);
         flash_actor.set_position (x, y);
 #if HAS_MUTTER47
-        flash_actor.set_background_color (Cogl.Color.from_string ("white"));
+        flash_actor.set_background_color (Cogl.Color.from_string ("#FFFFFF"));
 #elif HAS_MUTTER46
         flash_actor.set_background_color (Clutter.Color.from_pixel (0xffffffffu));
 #else

src/Widgets/SessionLocker.vala

@@ -121,7 +121,7 @@ public class Gala.SessionLocker : Clutter.Actor {
         });
 
 #if HAS_MUTTER47
-        background_color = Cogl.Color.from_string ("black");
+        background_color = Cogl.Color.from_string ("#000000");
 #else
         background_color = Clutter.Color.from_string ("black");
 #endif