Transfer all Textures to GPU at Load

[DavidMansolino]

Describe the Bug
Currently all the wren textures are loaded, setup and bind when the world is loaded, but OpenGL actually transfer them to the GPU only the first time they are used. Which can cause slowdowns while the simulation is running (because of culling new objects/textures can enter the scene, see for example https://www.gamedev.net/forums/topic/370069-texture-prefetch-preloading-solved/).

Steps to Reproduce
This is easily visible in big worlds when the viewpoint far field is set to non-zero. Moving the viewpoint initially produces jaggy motions and we can see the GPU memory increasing while we move. Then, once we have 'viewed' all the objects, the motion becomes smooth and GPU memory becomes stable.

Expected behavior
Performing an initial rendering without culling will force the transfer of all the textures to the GPU: this is currently in test here: cb98047