Expose shell to canvas::Program to match shader::Program.

[dtzxporter]

Allows a canvas program to schedule redraws as needed without managing it externally.

This matches the more powerful shader::Program update method now.

[bbb651]

For both of them, is there a reason Option<Message> is still needed into the return value if you have access to Shell? It's strictly more powerful because you can publish more than one message, and for other cases it seems unnecessary to have two ways to do the same thing. Widget already works like this, returning only Status

[hecrj]

This has already been addressed by #2662.