[svg] fix: when viewBox is absent, handle units such as 'mm'
[svg] fix: when viewBox is absent, handle units such as 'mm'

fixes https://github.com/twitwi/deck.js/issues/18
ref https://github.com/twitwi/deck.js/pull/16

twitwi/deck.js,twitwi/deck.js,twitwi/deck.js