Glk: Image scaling in buffer windows

[Dannii]

Going back to these:

@zarf

For the GlkOte JSON protocol, I am imagining these cases:

width:W, height:H    // the current usage

widthratio:R, height:H   // R is a float; 1.0 means 100% of the window width

width:W, heightratio:R   // R is a float; 1.0 means 1/1 aspect ratio

widthratio:R1, heightratio:R2

The first one will need to be converted to an aspect ratio for the final CSS (with a default maxwidth=100%).

The second one is the fixed height image, maxwidth is not used. This is the only one that would result in a height actually being set.

The third one is still not needed (it could just be sent in the first format), but would now be width:W, aspectwidth:AW, aspectheight:AH.

The fourth would now be widthratio:R, aspectwidth:AW, aspectheight:AH.

Will aspectwidth and aspectheight be integers or floats? imagerule_AspectRatio might result in a non-integer ratio, so floats are probably needed.

We’ll need a new protocol support flag.