iter5 Track E: move LAST_OUTPUT_WIDTH/HEIGHT from process-global to per-driver-data

Sonnet review 7.3 / 9.6 from iter1 + carried iter2/3/4 substrate.
Two libva driver_data instances in the same process (e.g. Firefox
playing two tabs at different resolutions, or Firefox + mpv via the
same dlopened backend) would race on the static cache.

Move to struct request_data.last_output_width/height. The V4L2
device fd is already per-driver_data, so this is the correct binding
unit (one fd, one current OUTPUT format).

Verified: two concurrent mpv processes (2s stagger) both decode
300 frames cleanly with no cross-corruption. Same-instant init still
hits kernel-level fd contention on /dev/video1 (hantro is a
single-instance device); cross-process serialization is out of scope
for a libva backend.

Resolves the surface_reset_format_cache() callsite: now takes
driver_data parameter (was zero-arg).

Also drops the 'rc' unused-variable warning in v4l2_ioctl_controls
that the iter5 sweep left behind.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/context.c

@@ -263,7 +263,7 @@ VAStatus RequestDestroyContext(VADriverContextP context, VAContextID context_id)
 	 * cached "already 1920x1088"); the exported descriptor encoded
 	 * wrong pitch/offset.
 	 */
-	surface_reset_format_cache();
+	surface_reset_format_cache(driver_data);
 
 	return VA_STATUS_SUCCESS;
 }