Don't pollute global namespace
No need for offscreen buffers to have a base class

This renders the render target base class obsolete
Rename offscreen buffer functions
Don't overwrite clear color
Rename drawViewAlignedQuad to drawQuad

... because that's what the function actually does
Unify vvRect and vvGLTools::Viewport

Viewport is now a typedef to vvRecti, is defined in vvrect.h
and resides in namespace virvo
Make vvgltools.h a genuine private header