Buffers in constant address space must be aligned to 256 bytes on macOS.
This fixes Metal validation errors:
Compute Function(cmdCopyBufferToImage3DDecompressDXTn): the offset into the buffer src that is bound at buffer index 0 must be a multiple of 256 but was set to 12928.
Compute Function(cmdCopyBufferToImage3DDecompressDXTn): the offset into the buffer src that is bound at buffer index 0 must be a multiple of 256 but was set to 78464.
1f8967a617