Reading and uploading texture data

[Aidan63]

Texture data is current immutable, once uploaded from the initial image it cannot be modified by the CPU, not can any changes made to it on the GPU be accessed by the CPU. Some means to read back a texture or upload new data should be added.
Not really sure how I want to go about this, uploading and downloading texture data is really expensive so I'm hesitant to add something which would be easy to misuse.