MetalPictureReader

Renders one picture offscreen through Metal and reads the RGBA bytes back (the hardware path on Apple). This is what replaces the CPU colour conversion for a consumer that needs BYTES rather than a layer, KiteVideo above all: the YUV-to-RGB arithmetic runs in the fragment shader, a VideoToolbox frame is wrapped with no copy at all, and the CPU pays exactly one readback memcpy, which is the no-CPU-RGBA law with the one copy its Android sibling also pays.

The picture comes back at its STORED size, unrotated and without the pixel-aspect stretch: the caller's own geometry (KiteVideo's draw phase) applies those, exactly as it did for the CPU converter's output.

NOT thread safe: one reader belongs to one worker thread, the same confinement every renderer in this library keeps.