The GPU module has 2 different styles when reading back data from
GPU buffers. The SSBOs used a memcpy to copy the data to a
pre-allocated buffer. IndexBuf/VertBuf gave back a driver/platform
controlled pointer to the memory.
Readback is done for test cases returning mapped pointers is not safe.
For this reason we settled on using the same approach as the SSBO.
Copy the data to a caller pre-allocated buffer.
Reason why this API is currently changed is that the Vulkan API is more
strict on mapping/unmapping buffers that can lead to potential issues
down the road.
Pull Request #104571
Implementation also contains a number of optimisations and feature enablements specific to the Metal API and Apple Silicon GPUs.
Ref T96261
Reviewed By: fclem
Maniphest Tasks: T96261
Differential Revision: https://developer.blender.org/D15369
