Only map memory that is meant to be accessed by CPU

[kvark]

Currently, gfx-memory unconditionally maps everything that is CPU-visible. It assumes that 1) mapping is harmless if we don't actually do anything with it, and 2) that on most systems non-cpu-visible memory types will be preferred for private resources.

However, on Intel GPUs apparently all memory types are CPU-visible. We don't want to map everything and exhaust the resident memory space. We don't want to confuse RenderDoc - baldurk/renderdoc#1863 (comment)

I wish we could approach this without breaking the API though.