Use minimal prebuilt FFmpeg for Windows

On the scrcpy-deps repo, I built FFmpeg 5.1.2 binaries for Windows with
only the features used by scrcpy.

For comparison, here are the sizes of the dll for FFmpeg 5.1.2:
 - before: 89M
 - after: 4.7M

It also allows to upgrade the old FFmpeg version (4.3.1) used for win32.

Refs <https://github.com/rom1v/scrcpy-deps>
Refs <https://github.com/Genymobile/scrcpy/issues/1753>

cross_win32.txt

@@ -16,9 +16,6 @@ cpu = 'i686'
 endian = 'little'
 
 [properties]
-ffmpeg_avcodec = 'avcodec-58'
-ffmpeg_avformat = 'avformat-58'
-ffmpeg_avutil = 'avutil-56'
-prebuilt_ffmpeg = 'ffmpeg-win32-4.3.1'
+prebuilt_ffmpeg = 'ffmpeg-5.1.2-scrcpy/win32'
 prebuilt_sdl2 = 'SDL2-2.26.1/i686-w64-mingw32'
 prebuilt_libusb = 'libusb-1.0.26/libusb-MinGW-Win32'