Improve writing of SVG files

[ezwelty]

• Clarify that SVG child elements are drawn in order, and thus from bottom to top layer as viewed in a vector editor like Adobe Illustrator
• Add <circle> element creator:

import xml.etree.ElementTree as ET
ET.Element('circle', cx=x, cy=y, **kwargs)

• Stringify non-string element attributes
• Add helper for scaling an :

SCALE = 0.25
glimpse.svg.svg(
  glimpse.svg.image(width=width, height=height, transform=f'matrix({SCALE} 0 0 {SCALE} 0 0)'),
  ET.Element('circle', cx=uv * SCALE, cy=uv * SCALE),
  width=str(width * SCALE),
  height=str(height * SCALE)
)