Replace the ImageInfo and PlacementInfo sized structs and their
associated .info enum variants with a new _get_multi pattern that
batches multiple enum+pointer pairs into a single call. This avoids
struct ABI concerns (field order, padding, alignment, GHOSTTY_INIT_SIZED)
while preserving the single-call-crossing performance benefit for FFI
and Cgo callers.
Each _get_multi function takes an array of enum keys, an array of
output pointers, and an optional out_written parameter that reports
how many values were successfully written before any error. This
applies uniformly to all _get APIs: terminal_get, cell_get, row_get,
render_state_get, render_state_row_get, render_state_row_cells_get,
kitty_graphics_image_get, and kitty_graphics_placement_get.
The C example is updated to use compound-literal _get_multi calls,
and tests cover both success and error paths for every new function.
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