[PATCH] doc: gpu: Add document describing buffer exchange

[Simon Ser]

> stride
> 	????

I think what's clear is:

- Per-plane property
- In bytes
- Offset between two consecutive rows

How that applies to weird YUV formats is the tricky question…

> Btw. there was a fun argument whether the same modifier value could
> mean different things on different devices. There were also arguments
> that a certain modifier could reference additional implicit memory on
> the device - memory that can only be accessed by very specific devices.
>
> I think AMLOGIC_FBC_LAYOUT_SCATTER was one of those.

A recent exmaple of this is [1].

[1]: https://patchwork.freedesktop.org/patch/452461/