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/render/renderer.h

@@ -12,6 +12,9 @@ struct RendererConfig {
     std::string window_title = "screencast receiver";
     int initial_width = 1280;
     int initial_height = 720;
+    // Borderless fullscreen. On the headless KMSDRM backend (no window
+    // manager) fullscreen is enabled automatically.
+    bool fullscreen = false;
 };
 
 class Renderer {