Have you ever wanted to decorate your builds with your very own Picasso or Van Gogh's, no worries, I'm not judging. I just wanted to let you know, that you totally can with the painting mod. You're welcome.
You can draw straight lines by holding down the shift key while painting.

LINKS:
Download Mod
Github repository
Forum Topic: Please visit this page to ask questions or request features.
P.S. You'll need the vector_extras mod to run this
TenPlus1 made an update to this that you can download here.