core (image): convert SVG images + better error handling: - when an SVG image is found, new backend `imageConvert` method is used to convert it to PNG - by default, 128x128 is used as dest size. - removed code to handle image without extension in filename, as it is now working with Kivy. - if texture is not updated after `texture_update` is called, an error is logged, a fallback image is used, and `on_error` event is dispatched

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .