perf(codec): upgrade x264 preset and downscale to the receiver display

Two quality improvements:

Preset: ultrafast -> veryfast. Unlocks Main profile with CABAC
entropy coding, hexagonal motion search, 3 reference frames, and
adaptive quantization — typically 30-40% better quality at the same
bitrate. The desktop handles the extra encoding cost trivially
(150+ fps at 1080p).

Downscaling: the receiver now advertises its display resolution in
the signaling answer (display_width/display_height, 0 = unknown).
When the capture exceeds the display (e.g. 2256x1504 source on a
1920x1080 receiver), the sender scales down preserving aspect ratio
before encoding — the same sws_scale pass that already converts the
pixel format also handles the resolution change, so there is no
extra step. This concentrates the entire bitrate into pixels the
display actually shows (~2.7x more bits per visible pixel at
4000 kbps when going from 2256x1504 to 1620x1080).

The renderer caches the display size during window creation
(native monitor resolution in fullscreen/KMSDRM; window size
otherwise). The receiver includes it in every signaling answer; the
sender pipeline computes aspect-preserving, even-rounded scaled
dimensions when the display is smaller than the capture.

meson test 5/5 in both configurations, valgrind clean.

include/screencast/render/renderer.h

@@ -24,6 +24,11 @@ class Renderer {
     // Present one decoded frame. Returns false if the window was closed.
     virtual bool present(const DecodedFrame& frame) = 0;
 
+    // The display resolution (native monitor size in fullscreen mode).
+    // Used by the receiver to tell the sender what resolution to encode at.
+    virtual int display_width() const = 0;
+    virtual int display_height() const = 0;
+
     // Pump events (window close, resize). Non-blocking.
     virtual bool poll_events() = 0;