renderer: reduce image streaming overhead (#1467)

Reduce frame preparation and transport costs so local terminals can
consume image updates with less CPU work. Bound pending frames when
output stalls and fix per-frame GPU resource growth so sustained
rendering does not accumulate work or retain native handles.

Keep raw transport as the default and make file transport opt-in for
terminals that acknowledge image uploads.

#1328 partially superseded, but it has `NativeImage.adoptRgbaFile(path,
width, height)` that adopts an existing producer-created RGBA file
lazily. Eligible renders send that same path without reading, copying,
or rewriting its pixels.