drawable: Draw bgimage with operator OVER

The actual bg is drawn either with fake transparency over the wallpaper (this
uses operator OVER) or for true transparency with operator SOURCE. The bgimage
should be drawn ontop of this without erasing the background and thus needs
operator OVER.

However, before this commit the bgimage was drawn in the same way as the bg and
thus inherited its SOURCE operator if a compositor is running. Fix this by
restoring the default operator (OVER) and also e.g. the default source before
drawing the bgimage.

Fixes: https://github.com/awesomeWM/awesome/issues/954
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/drawable.lua

@@ -123,8 +123,11 @@ local function do_redraw(self)
     cr:set_source(self.background_color)
     cr:paint()
 
+    cr:restore()
+
     -- Paint the background image
     if self.background_image then
+        cr:save()
         if type(self.background_image) == "function" then
             self.background_image(context, cr, width, height, unpack(self.background_image_args))
         else
@@ -132,10 +135,9 @@ local function do_redraw(self)
             cr:set_source(pattern)
             cr:paint()
         end
+        cr:restore()
     end
 
-    cr:restore()
-
     -- Draw the widget
     if self._widget_hierarchy then
         cr:set_source(self.foreground_color)