fix(render): use the correct SDL pixel format for FFmpeg RGBA

DecodedFrame pixels are AV_PIX_FMT_RGBA (memory order R,G,B,A), but
SDL names 32-bit formats MSB-first, so SDL_PIXELFORMAT_RGBA8888 reads
memory as A,B,G,R: the opaque alpha byte was displayed as red and the
image got a strong red tint. The correct constant is
SDL_PIXELFORMAT_ABGR8888.

Verified with solid red/green/blue synthetic RTP feeds through the
real receiver: each hue now lands on its reference YUV values
(red Y79/U87/V247, green Y148/U48/V27, blue Y34/U248/V108), where
green previously displayed as magenta.

src/render/sdl_renderer.cpp

@@ -7,9 +7,10 @@
 namespace sc {
 namespace {
 
-// DecodedFrame pixels are AV_PIX_FMT_RGBA: memory order R, G, B, A, which
-// matches SDL_PIXELFORMAT_RGBA8888.
-constexpr SDL_PixelFormat kSdlPixelFormat = SDL_PIXELFORMAT_RGBA8888;
+// DecodedFrame pixels are AV_PIX_FMT_RGBA: memory order R, G, B, A. SDL
+// names 32-bit formats MSB-first, so that byte order is SDL's ABGR8888 —
+// using RGBA8888 would read the alpha byte as red and tint the image red.
+constexpr SDL_PixelFormat kSdlPixelFormat = SDL_PIXELFORMAT_ABGR8888;
 
 class SdlRenderer final : public Renderer {
   public: