Change the default pixel format from RGB to RGBA for render texture when saving as PNG.
Make the API more extendable
The filename should be converted to lower case before comparing.
Merge pull request #7056 from visiblelight/render_texture

Use RGBA when saving a file from render texture
[ci skip]
[AUTO]: updating luabinding automatically
Merge pull request #7075 from CocosRobot/update_lua_bindings_1402886913

[AUTO]: updating luabinding automatically