fix: YUV420 buffer overflow for odd-dimension frames in VideoBuilder
The UV plane size was calculated as ySize/2 (integer division), but the
UV loop iterates ceil(w/2)*ceil(h/2) times writing 2 bytes each. With
odd width or height the loop writes past the allocated buffer, causing
an ArrayIndexOutOfBoundsException.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>