Ability to clear a container

[tristil]

Hi, thanks for this library!

I'm porting some Curses-based code over from Ruby to use termbox-crystal, here. I know this is still early days for this library but I thought it might be useful to share some of the higher-level functions I'm implementing in my code, in case you want to implement something like them.

For example, here is some code I've written to in effect let a container clear its contents. I'm not recommending the actual code, which is convoluted, but the idea of a method to replace all the cells of a container with blank characters:

https://github.com/tristil/minder-ui/blob/master/src/minder/frame.cr#L70

A cleaner implementation for what I've done here would be based on a matrix of positions stored on the container. This Go library uses buffers of cell positions this way.

Anyway, hope this kind of thing is useful. I know that the interface for the high-level aspects of this library still has to be fleshed out, but it works well enough now for me to build my app.