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

f95d0c31d45558c7b3e2fc39398f51eb0190bdca 0.7.0a1
4233be74572a1af8eebd502c009a10641c63ca0c 0.7.0a3
5c28cf8815319abd8c963022091cf4726764a3e6 0.7.0b1
51f73d210da908c9fb292e6bd7fda42a9d2b8453 0.7.0