For image (sixel, iTerm2, Kitty...) support that handles graphics in
terms of `Rect` so that the image area can be included in layouts.
For example: an image is loaded with a known pixel-size, and drawn, but
the image protocol has no mechanism of knowing the actual cell/character
area that been drawn on. It is then impossible to skip overdrawing the
area.
Returning the window size in pixel-width / pixel-height, together with
colums / rows, it can be possible to account the pixel size of each cell
/ character, and then known the `Rect` of a given image, and also resize
the image so that it fits exactly in a `Rect`.
Crossterm and termwiz also both return both sizes from one syscall,
while termion does two.
Add a `Size` struct for the cases where a `Rect`'s `x`/`y` is unused
(always zero).
`Size` is not "clipped" for `area < u16::max_value()` like `Rect`. This
is why there are `From` implementations between the two.
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