Simple drawing canvas

A simple framework for drawing on screen.

Key features:
* draw directly on the texture
* different pen sizes and colors, easy to configure more
* save/load the texture from Application.persistentDataPath
* works on mobile (tested on Android and iOS)
* full commented source code included
* easy to change to suit your needs
* clear to a specific color or transparent
* "stickers" (the ability to draw a texture at specific coordinates on the drawing canvas) * draw various shapes (circle, rectangle, polygon)