pane: drop the render snapshot of panes on hidden tabs -- 154K -> 66K each

An updated GhosttyRenderState retains a full copy of the pane's viewport
(rows, cells, per-row arenas, styles) so renders can read it without the
terminal lock. Measured with bpftrace and a lib-vt micro-benchmark, that
copy is ~140KB for an 80x24 pane -- more than twice the terminal it
mirrors (~57KB) -- and every pane kept one forever, composed or not.

Only the current tab is ever composed, so app_compose now drops the
snapshot of every pane on the other tabs (a pointer test once dropped),
and pane_new no longer creates one at all: pane_compose and
pane_select_word build it on first use. Both already begin with
ghostty_render_state_update(), so a dropped snapshot is rebuilt, never
missed, and the tab switch already forces a full repaint.

Server memory per empty 80x24 terminal drops from 154K to 66K anon
(Superlogical reports 68K on macOS; tmux 4K, zellij 1.2-6.2M). Fifty
background tabs: 8.2M -> 3.8M, reclaimed on close, and cycling through
tabs does not ratchet. What remains is the lib-vt terminal itself.

1 files changed, 4 insertions, 0 deletions

diff --git a/include/slosh.h b/include/slosh.h
index 47d8220..6c95426 100644
--- a/include/slosh.h
+++ b/include/slosh.h
@@ -261,6 +261,10 @@ bool pane_dirty(pane_t *p);
 /* Ask for a repaint without having changed a cell: what is drawn *over* the
  * pane changed, and composing is the only thing that can see that. */
 void pane_touch(pane_t *p);
+/* Free the pane's retained render snapshot (~140KB for 80x24). Safe at any
+ * time: the next render rebuilds it from the terminal. Called for panes on
+ * tabs that are not on screen. */
+void pane_render_cache_drop(pane_t *p);
 /* Scrollback. Negative delta scrolls up (towards older output). */
 void pane_scroll(pane_t *p, int delta);
 void pane_scroll_edge(pane_t *p, bool top);