Texture callback

[sammyfreg]

As described in issue #8.

When parsing the drawing commands, would be nice to auto-detect missing textures and let user know about them (using a callback). They could then send the data the the netImgui server. This would make it so less code change would be required in original user codebase, could be centralized into the callback.

However, this is not trivial, since user still need access to texture data, and often will have been release from CPU side memory and only reside on the GPU.