1.3.2: Preserve HDR when upscaling instead of crushing to SDR (#36)

HDR (PQ/HLG) inputs were decoded, upscaled, and encoded through an 8-bit
BGRA pipeline, so the output was tagged BT.2020/PQ but only contained 8-bit
Main-profile data - effectively downscaling HDR to SDR (#30).

Detect PQ/HLG transfer functions and route those inputs through a 16-bit
half-float (RGBA16Float) pipeline end to end: decode to 64RGBAHalf, run
MetalFX at rgba16Float, and encode HEVC Main10. Mastering display and
content light level metadata are forwarded to the encoder. SDR content is
unchanged and stays on the fast 8-bit path.