Deltas: change line join to "round"

-otherwise the drawn line joins become to ugly corners
client: draw the full line on paint preview

-the client is painting a preview before sending it to the server
-this preview is now generated by drawing the whole line on every mouse move event, not only the last segment of the line
-fix the issue when using an opacity < 1.0 : the line segments will not overlapping anymore
client: remove transparent background poster

-replaced by CSS style ("background" attribute)
update TODO.md