Convert CSS colour to standard hex colour (with alpha0)

CSS colours can be lots of things:

css_colour_to_hex(x)

Arguments

x	CSS colour

hex colour (always with alpha channel)

Hex colour with 3, 6 or 8 characters
CSS colour name e.g. 'red'. Not all CSS colours correspond to R colours e.g. 'silver' is not in R
rgb(), hsl(), hcl(), lab() and other colourspace-specific colour constructores. Only rgb and hsl are currently handled
color() for complex colourspace specifidation. Not handled yet
'transparent' and sometimes 'none' to indicate '#00000000'
'currentColor' or 'currentcolor' to indicate the colour for this element should be taken from whatever the current 'color' property is
for SVG, colours colours can be references to 'linearGradient' and 'radialGradient' specifications, or patterns