ffmpeg_bridge: fix cross-platform CI failures (Ubuntu/macOS/Windows)

Ubuntu (FFmpeg 7.0) build failure: the public FB_PIX_FMT_* values were
hardcoded to AVPixelFormat enum values, but those shift between FFmpeg
releases (new formats are inserted mid-enum: X2RGB10 in 7.0,
RGBF16/GRAYF16 later), so the static_asserts in internal.h failed and
RGBF16LE/GRAYF16LE did not exist at all.

The FB pixel format values are now fixed identifiers translated inside
the library by pixel format name (stable across releases):
- fb::PixFmtToAV/PixFmtFromAV resolve the static table by name;
  formats a decoder produces that have no static identifier (hardware
  downloads like nv12/p010le/p210le) receive process-local dynamic ids
  >= 1000 so they keep round-tripping through the API (scaler etc.)
  instead of collapsing to NONE and crashing sws.
- All crossing points translated: scaler, frame get/set format,
  decoder/probe stream info, encoder config and write_video_frame,
  fb_pix_fmt_* utilities, fb_find_best_pix_fmt_of_list (which also
  reinterpret_cast the caller's list; now translated element-wise and
  properly NONE-terminated).
- nv12 and p010le get static identifiers as the common hardware
  download formats.
- Hardware frame detection no longer compares pixel formats (breaks
  across format spaces); it uses the existing fb_frame_is_hw().

macOS test failures (FFmpegBridgeEncoder.WritePngVideoAndProbeBack,
WritePcmAudioAndProbeBack): config.filename held a dangling pointer to
a temporary QByteArray (UB; happened to work on Linux). Keep the
QByteArray alive for the encoder's lifetime.

Windows (MSYS2 UCRT64) build failure: timecodefunctions.h used int64_t
without including <cstdint> (previously pulled in transitively).

Full gtest suite: 584 passed.

ffmpeg_bridge/src/sws.cpp

@@ -30,8 +30,8 @@ FBScaler *fb_scaler_create(int src_width, int src_height, int src_format,
 						   int flags)
 {
 	SwsContext *ctx = sws_getContext(
-		src_width, src_height, static_cast<AVPixelFormat>(src_format),
-		dst_width, dst_height, static_cast<AVPixelFormat>(dst_format), flags,
+		src_width, src_height, fb::PixFmtToAV(src_format),
+		dst_width, dst_height, fb::PixFmtToAV(dst_format), flags,
 		nullptr, nullptr, nullptr);
 	if (!ctx) {
 		return nullptr;