programmable graphics: introduce triangles (perhaps more general pictures)

Semantically, a picture is a function from co-ordinate to Bool, but this general way of construction is not available via the AST. Only way to build a picture is to use Rectangle and Circle.

We could add more names for elementary pictures, e.g., Triangle for \ (x,y) -> x>=y

Long-term, add AST syntax for building general pictures: this needs reified arithmetic and relational expression (co-ordinates are numbers, pixels are Booleans). We probably add these anyway when we do something about control flow (if-then-else needs Booleans, iteration needs numbers) but let's do this in a separate issue.