Add draw to image operations, possibly as environment.

[foresterre]

Playground:

# begin := start a scoped environment
# end := end the scoped environment
# 
# initially only in the root (i.e. no true scoping)
begin draw:
  # commands only valid within the draw environment
  move-cursor 100 200;
  color #fff000;
  cursor-down;
  step 100 north;
  rect <...>
  quad <...>
end;

• internal name: Statement::Scope?

inspiration:
processing

• https://processing.org/reference/
• https://p5js.org/reference/

[foresterre]

The imageproc crate already has many drawing functions built-in:

• https://github.com/image-rs/imageproc/blob/master/examples/drawing.rs
• https://docs.rs/imageproc/0.18.0/imageproc/drawing/index.html

Relevant for #99