renderable: add xzgrid and fullscreenquad renderables
core: add FrameBuffer object for render-to-texture
rectangle.glsl - various effects for copy-texture-to-screen
hello: don't rely on app to clear framebuffer and enable depthtest
object: optional post-proccessing (toggle with SPACE)
app: don't clear framebuffer and enable depth testing

apps may have different rendering needs. let them decide.