[PATCH weston] compositor: Fix rendering with shm transformed buffers

[Rob Bradford]

> Say you have a 500x250 surface, but its buffer is rotated 90 degrees, so
> that it is 250x500. If the surface damage is the rectangle defined by the
> points (10, 20) and (40, 50), we would convert both coordinates and have
> (230,10) and (200,40). This does define a rectangle in the buffer, but if we
> calculate width as x2 - x1, we would get a negative value.
>
Hi Ander, thanks for the classifications,

It passed me by that since all the transformations are based on
rotations about 90 degrees then that would still land on exact pixel
boundaries.

Cheerio,

Rob