sometimes we need to ensure memory allocation has a particular alignment
in an _offset_ buffer (which we have no control over, eg renderdoc's
OpenGL buffers).
this applies an offset to various operations that make the
aligned::direct allocator correctly align allocations for buffers that
aren't themselves aligned.
this allows the users to more easily walk the byte ranges (or perform
simply pointer arithmetic), without as much danger of using the values
in an expression inadvertantly.
