gears.surface: Change the way that caching works

Loading a file normally has the same behaviour as before. First the cache is
checked and if nothing is found, the file is loaded and cached.

This commit changes the behaviour of loading a file uncached. This no longer
removes the file from the cache if it is cached (why should it?) and also does
not put it in the cache.

This means that users of load_uncached and load_uncached_silently can now freely
modify the resulting surface without interfering with other API users.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/surface.lua

@@ -41,8 +41,6 @@ function surface.load_uncached_silently(_surface, default)
     if not _surface then
         return get_default(default)
     end
-    -- Remove from cache if it was cached
-    surface_cache[_surface] = nil
     -- lgi cairo surfaces don't get changed either
     if cairo.Surface:is_type_of(_surface) then
         return _surface
@@ -57,12 +55,7 @@ function surface.load_uncached_silently(_surface, default)
         end
     end
     -- Everything else gets forced into a surface
-    _surface = cairo.Surface(_surface, true)
-    -- If we loaded a file, cache it
-    if file then
-        surface_cache[file] = _surface
-    end
-    return _surface
+    return cairo.Surface(_surface, true)
 end
 
 --- Try to convert the argument into an lgi cairo surface.
@@ -80,6 +73,12 @@ function surface.load_silently(_surface, default)
         if cache then
             return cache
         end
+        local result, err = surface.load_uncached_silently(_surface, default)
+        if not err then
+            -- Cache the file
+            surface_cache[_surface] = result
+        end
+        return result, err
     end
     return surface.load_uncached_silently(_surface, default)
 end