missing SVG ViewBox -> SVGs don't resize in browser

[jaberg]

SVGs included via svg_include should include a viewBox, which will allow them to be resized (at least downsized) properly. showboat could be helpful in either

• warning about SVGs that do not have a viewBox
• inserting viewbox="0 0 width height" to make them work nicely.

[davidcox]

Good point. Either of these would be a good things to add.