gears.surface: Cache files from disk

Instead of loading files from disk every time we need them, add a cache to
gears.surface as a weak table that maps strings to cairo surfaces.

If this cache should be avoided, there is a new gears.surface.load_uncached()
function which works just like gears.surface.load() worked before.

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

lib/gears/surface.lua.in

@@ -17,14 +17,17 @@ end
 
 -- gears.surface
 local surface = { mt = {} }
+local surface_cache = setmetatable({}, { __mode = 'v' })
 
 --- Try to convert the argument into an lgi cairo surface.
 -- This is usually needed for loading images by file name.
-function surface.load(_surface)
+function surface.load_uncached(_surface)
     -- Nil is not changed
     if not _surface then
         return nil
     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
@@ -37,6 +40,18 @@ function surface.load(_surface)
     return cairo.Surface(_surface, true)
 end
 
+function surface.load(_surface)
+    if type(_surface) == "string" then
+        local cache = surface_cache[_surface]
+        if not cache then
+            cache = surface.load_uncached(_surface)
+            surface_cache[_surface] = cache
+        end
+        return cache
+    end
+    return surface.load_uncached(_surface)
+end
+
 function surface.mt:__call(...)
     return surface.load(...)
 end