modified renderPasses to allow it to render into a supplied texture

Marked calculateBounds as protected to allow inherited classes to make
use of its calulations.

Removed assertion if a renderTarget is supplied in the RenderSupport
object; and tweaked the final pass code to redirect output to the
supplied renderTarget.

I think this also addresses #391; or at least part of it
Bloom (fog) filter

Uses the Blur filter; rendering it to a texture.  Then blends the two
using the lighten mode