html: color(display-p3 ...) support

[jms72]

When using jsPDF's html function on a DOM element containing display-p3 colors it crashes with:
Attempting to parse an unsupported color function "color"

The css is: background-color: color(display-p3 0.778 0.688 0.983)

Maybe you can convert the p3 into RGB?

Also after the crash, the page is overlayed with html2pdf__overlay blocking page interactivity, and some other temporary iframes are left, which could be cleaned up on error.