feat(render): fullscreen headless receiver with letterboxing

A headless receiver has no window manager, so a windowed window is
meaningless there — the screencast should simply fill the screen. The
renderer now goes borderless fullscreen automatically when SDL runs
the KMSDRM backend, and --fullscreen forces the same behavior in
desktop sessions. The video keeps its aspect ratio via
SDL_SetRenderLogicalPresentation(LETTERBOX): a 4:3 desktop on a 16:9
TV renders with black bars instead of a stretched picture, the cursor
is hidden, and the clear-before-draw keeps the bars black.

Verified live on a 3440x1440 monitor: the --fullscreen receiver window
covered the entire display while a 4:3 synthetic feed rendered with
pure-black pillarbox bars (Y=0/SAT=0) and a saturated-red center
(V=254). meson test 5/5 in both build configurations.

docs/RUNBOOK.md

@@ -102,6 +102,11 @@ the recommended setup for small boards (Raspberry Pi OS **Lite**):
 3. Disable console blanking: append `consoleblank=0` to
    `/boot/firmware/cmdline.txt` and reboot.
 
+Under KMSDRM the receiver goes **fullscreen automatically**, with
+aspect-preserving letterboxing (a 4:3 desktop on a 16:9 TV shows black
+bars, not a stretched picture) and no mouse cursor. On desktop sessions
+`--fullscreen` forces the same behavior.
+
 A window manager only comes with the desktop-session alternative, where
 the receiver runs inside it (uncomment the `Environment=` lines in the
 service file as described above).