Implement :shrink()

[viluon]

Implement a method for shrinking a buffer using the teletext characters of CC 1.76+.

[viluon]

Huh, that's quite inefficient though - isn't it? We wouldn't need a buffer with 3 pixel properties, we'd only utilise the background colour. Therefore pixels could only be numbers instead of arrays - we need a new buffer library for this, or somehow branch the existing buffer... I know @exerro solved this by using a preprocessor (which is both ugly and against the Desktox development process philosophy). So it'd probably be best to just have another module, but that's twice the code to maintain :/

[viluon]

Shrinking should be a feature of another (external) library, and is outside the scope of desktox.buffer. Rendering a pixel-perfect buffer to a terminal buffer should "shrink" automagically, so the functionality will essentially be there, once pixel-perfect is implemented.

[viluon]

Actually, thanks to #16, we can resolve #10, for which we'll need :shrink().