<symbol> width/height

[edemaine]

<symbol viewBox="0 0 10 10" width="20" height="20"> should create a symbol that renders as final size 20x20, but currently width/height are ignored by SVG Tiler and it gets rendered as 10x10.

The behavior we want is:

• Absent width/height inherits width/height from viewBox. (In SVG, it would actually default to auto which means 100%, but this default seems useless, and also very different from current SVG Tiler behavior.)
• width/height explicitly set to auto makes for the auto-resizing behavior.
• (All bullets above are technically deviations from SVG spec.)
• Numeric width/height get treated as in SVG, with units translated to px for SVG coordinates.
• Absent viewBox becomes 0 0 <width> <height> if width and height are present (no automatic bounding box computation).
• Actually, if we properly set width/height on all <symbol> tags, we can omit width/height in use statements (except in auto mode), which would save a lot of characters (more re-use!).