Day 31: Position fro depth, part 1

Today i started working on another big optimization, which is to build back the position of the g-buffer fragments from their depths.

This allows to use just the depth texture instead of a 4 32 bit texture for position.

Probably i'll need a second depth texture, because it's content is overriden during the stencil pass of the point ligths, but that's still 1/4 of the size of the position texture.

I started today with this process, but it is not working yet, i'm getting strange stuff on screen. I hope to finish tomorrow