Removes vsOverlay from VectorStorm.

We haven't supported overlays since VectorStorm was a 2D engine.  They
were a quirky, fun idea.  But not actually performant or useful for real
games.  And they haven't worked in years, anyhow, so I'm finally
removing their old source files and the code which used them.
Fixes stencil rendering for bloom/shader render schemes.

Offscreen render buffers weren't having their stencil buffers cleared.
Oops.
Cleanups to bloom rendering.

Mostly removing old, commented-out code.