Avoid memory invalidation when writing

Rendy's [map](https://docs.rs/rendy-memory/0.5.2/rendy_memory/trait.Block.html#tymethod.map) method tries to be too smart and ends up invalidating the range, even when the user simply intends to write data to it. Invalidation is not free - https://github.com/gfx-rs/gfx/issues/3099 , but most importantly - we should have a way to avoid it here entirely.

One idea is to have the user specifying explicitly what the memory is mapped for: read, write, or both. That requires API changes, unless you want to expose this as a separate method (even something like `map_discard` would do).