add implementation of a container widget with alpha-blitting

This is a container that will honor a given alpha value (0..1)
when painting, so it will be translucent for values < 1.

frontend/ui/widget/container/alphacontainer.lua

@@ -0,0 +1,54 @@
+local WidgetContainer = require("ui/widget/container/widgetcontainer")
+local BlitBuffer = require("ffi/blitbuffer")
+
+--[[
+AlphaContainer will paint its content (1 widget) onto lower levels using
+a transparency (0..1)
+--]]
+local AlphaContainer = WidgetContainer:new{
+    alpha = 1,
+    -- we cache a blitbuffer object for re-use here:
+    private_bb = nil,
+    -- we save the underlying area here:
+    background_bb = nil,
+    background_bb_x = nil,
+    background_bb_y = nil
+}
+
+function AlphaContainer:paintTo(bb, x, y)
+    local contentSize = self[1]:getSize()
+    local private_bb = self.private_bb
+
+    if self.background_bb then
+        -- we have a saved copy of what was below our paint area
+        -- we restore this first
+        bb:blitFrom(self.background_bb, self.background_bb_x, self.background_bb_y)
+    end
+
+    if not private_bb
+    or private_bb:getWidth() ~= contentSize.w
+    or private_bb:getHeight() ~= contentSize.h
+    then
+        -- create private blitbuffer for our child widget to paint to
+        private_bb = BlitBuffer.new(contentSize.w, contentSize.h, bb:getType())
+        self.private_bb = private_bb
+
+        -- save what is below our painting area
+        if not self.background_bb
+        or self.background_bb:getWidth() ~= contentSize.w
+        or self.background_bb:getHeight() ~= contentSize.h
+        then
+            self.background_bb = BlitBuffer.new(contentSize.w, contentSize.h, bb:getType())
+        end
+        self.background_bb:blitFrom(bb, 0, 0, x, y)
+    end
+
+    -- now have our childs paint to the private blitbuffer
+    -- TODO: should we clean before painting?
+    self[1]:paintTo(private_bb, 0, 0)
+
+    -- blit the private blitbuffer to our parent blitbuffer
+    bb:addblitFrom(private_bb, x, y, nil, nil, nil, nil, self.alpha)
+end
+
+return AlphaContainer