Rewrite texture manager

[Narann]

The main bottleneck is rice is texture management. It make some games hangs a lot. It looks like PurgeOldTextures() takes a lot of time.

I took a look at the code, and understood a little more how it work (and why it's irrelevant).

It has been written during an era where the whole GPU memory where the framebuffer (8MB) so you have to store textures on the RAM so the texture caching system fundation is highly related to this old model.

I still have to think about how design a potential new texture mangement system (it's a massive job...). I wonder how Dolphin handle this. I will have a look at their code.