OGL StreamBuffer optimization

OGL::StreamBuffer was very high in profilation (2.5%), so here are some optimization for it.
- Alloc fences directly in the StreamBuffer class, so they can't alias other member variables
- Align the size by power of two and use shifting instead of divisions for slot calculation
- Move alignment to the caller, so it can be skipped if not needed (eg indexbuffer)
- Reduce index size to 32bit, faster alignment division