Imported from Adélie. Firefox is prone to this warning on BE and
it leads to huge .xsession-errors, and the warning is not very
important or meaningful, so just disable it.

--- gfx/2d/HelpersCairo.h
+++ gfx/2d/HelpersCairo.h
@@ -147,7 +147,7 @@
     case SurfaceFormat::R5G6B5_UINT16:
       return CAIRO_FORMAT_RGB16_565;
     default:
-      gfxCriticalError() << "Unknown image format " << (int)format;
+      //gfxCriticalError() << "Unknown image format " << (int)format;
       return CAIRO_FORMAT_ARGB32;
   }
 }