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

[kwokcb]

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