[avisynth] Document that AddFunction won't work with ctypes
Update the video preview when changing a 'YUV -> RGB' option without re-evaluating the script
Allow to preview some stacked or interleaved high bit depth formats

Currently,
yuv420p10le, yuv444p10le, gray10le
yuv420p16le, yuv444p16le, gray16le

video menu -> bit depth

Important: this commit introduces changes needed to display a preview
clip with different resolution than the real one. Some old related code
was removed in 0062a260

Also to note is that the GetVideoWidth and GetVideoHeight macro functions
will still return the original dimensions