libghostty: expose Kitty graphics virtual placements (Unicode placeholders) to embedders

[nckre]

Ghostty renders Kitty graphics Unicode placeholders (a=p,U=1 + U+10EEEE cells) fine, but libghostty does not.

Tried this in Rex where direct placements (a=T, and a=t followed by a=p) render correctly but placements using Unicode placeholders don't. The cells show as raw glyphs and no image is drawn.

This matters e.g. for TUI apps like terminal-browser which shows a blank pane in Rex, while its page loads and takes input normally.

Gaps seems to be according to Claude review:

include/ghostty/vt/kitty_graphics.h can identify a virtual placement (GHOSTTY_KITTY_GRAPHICS_PLACEMENT_DATA_IS_VIRTUAL), but the geometry calls return nothing for one:

• ghostty_kitty_graphics_placement_rect: "Virtual placements (unicode placeholders) return GHOSTTY_NO_VALUE."
• ghostty_kitty_graphics_placement_viewport_pos: returns GHOSTTY_NO_VALUE "when the placement is a virtual (unicode placeholder) placement."

So an embedder can see that a placeholder image exists but can't work out which cells it covers or which part of the image belongs in each cell. #12111 exposed image data through the render state; this request is the remaining piece for virtual placements.