Fix #111703: banding and color shift when decoding some videos

Tell ffmpeg swscale to do accurate YUV->RGB conversion, instead of
slightly faster but not really accurate one. Fixes banding and some
color shifts in video files, particularly in dark regions.

The accurate conversion is a bit slower though, on 4K resolution video,
time taken to convert video frame from YUV to RGB:

- x64 (Ryzen 5950X): 2.3ms -> 3.7ms
- arm64 (M1 Max): 0.6ms -> 2.9ms

My take is that paying 1-2ms per 4K video playback is acceptable
since the result is obviously "more correct" and matches what VLC/ffplay
produces.

From what I can tell, "accurate conversion" turns off some dedicated
assembly code paths within ffmpeg. Maybe someday ffmpeg would get
accurate and assembly-optimized routines for that.

With more accurate decoding, we can now lower the expected render
test threshold again, since x64 & arm64 decoding is much closer now.

Comparison screenshots in the PR.

Pull Request: https://projects.blender.org/blender/blender/pulls/130383

tests/python/sequencer_render_tests.py

@@ -50,7 +50,7 @@ def main():
     report = render_report.Report("Sequencer", output_dir, oiiotool)
     report.set_pixelated(True)
     # Default error tolerances are quite large, lower them.
-    report.set_fail_threshold(3.0 / 255.0)
+    report.set_fail_threshold(2.0 / 255.0)
     report.set_fail_percent(0.01)
     report.set_reference_dir("reference")