`Rect` documentation does not specify Origin location

[noahprice-dev]

In the existing Rust-SDL2 documentation, the specific origin of a created rectangle is not specified. In the SDL2 documentation it specifies rectangles have an origin in the upper left:
https://wiki.libsdl.org/SDL2/SDL_Rect

A rectangle, with the origin at the upper left (integer).

Assuming this is still correct based on how we are calling the SDL library, I propose we add this information to the Rect::new() Doc comment string at the very least, and as well add a method on Rect to access this position for completeness. This would be similar to Rect::center()

[Cobrand]

Closed by #1493