TextureBaker: No way to create baked documents / images without writing to disk

## Issue

Currently the texture baker API does not allow access to the resulting baked documents and images in memory.
They must be written to disk and re-read in again which is inconvenient where file access is either inconvenient or
not available.

## Proposal

Allow to not always write to disk and provide accessors to the in-memory images and documents.
Basically this members:
* _bakedImageMap
* _bakedTextureDoc

Start with the TextureBaker API and inspect the existing _bakedImageMap and _bakedTextureDoc members. Trace how baked documents and images are currently written to disk, then define the in-memory access path and optional disk-writing behavior. Done means callers can access the baked results without requiring file access.

Written by the indexing model from the issue text.