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.

include/screencast/app/cli.h

@@ -17,6 +17,7 @@ struct ReceiveCommand {
     std::string_view peer_address; // optional, informational
     int local_rtp_port = 5004;
     int signaling_port = 5005;
+    bool fullscreen = false;
 };
 
 struct DiscoverCommand {