Simplify sprite build
Fix #466 (closed).
Tested with Firefox, Chrome and Konqueror (WebKit engine), I can't on Safari or InternetExplorer. But if https://up.nicofrand.eu/test_svg/sprite.svg#bank1 only displays a house and https://up.nicofrand.eu/test_svg/sprite.svg#bank2 only displays a pig, that should be fine (for quicker tests).