macOS: pick the YCbCr matrix per buffer instead of hard-coding BT.709 #1420

On macOS, sample_yuv in crates/compositor/src/shaders.metal always applies BT.709 coefficients. The buffer marker only selects the range, so a BT.601 VideoToolbox buffer renders with slightly wrong hues. The matrix should come from each buffer's YCbCr matrix attachment.

Pre-existing; spotted by CodeRabbit while reviewing #1408. Refs #1408