Expand PostFX Viewport Options

- Added an option for a postFX to get its viewport from a named texture
in slot 0, if there is one.  This allows the postFX to operate when the
named input texture's viewport is different than the current viewport.
- Modified the SSAO postFX to use the new
PFXTargetViewport_NamedInTexture0 option to more closely link SSAO with
the prepass buffer.
- Modifed the GFX method setActiveRenderTarget() with a new parameter
that indicates if the current viewport should be modified with the new
rendering target.  This defaults to true to maintain its previous
behaviour.  The postFX rendering pipeline sets this to false as it now
handles its own viewport setup, and removes an unnecessary
GFX->setViewport() call.