img: Add back support for SVG images, bypassing ImageMagick and simply passing the...

img: Add back support for SVG images, bypassing ImageMagick and simply passing the SVG through to the browser

SVG scaling by img directives has subtly changed; where before size=wxh
would preserve aspect ratio, this cannot be done when passing them through
and so specifying both a width and height can change the SVG's aspect
ratio.

(This patch looks significantly more complex than it was, because a large
block of code had to be indented.)

[smcv: drop trailing whitespace, fix some spelling]