This also separates the blend-to-screen part of render_layer into a
helper function, as that will be useful in the future with public layer
elements.
Relates to #725
resvg produces an image with pre-multipled alpha and anecdotal evidence suggests that HTMLImageElement on an SVG does, too.
This might need further restriction for the HTMLImageElement case just to svg, but
let's try this.
The README.md contains the warning that used to be in lib.rs.
Add README.md files to all internal crates
... pointing to the official public crate to use instead.
Rename internal crates
fixup: README files
fixup rename
