remove all warnings in hoodle-render
refactoring in Graphics.Hoodle.Render.Background
when rescaling pdf page, send surface id.
move pdfRendererMain to hoodle-render. implement genRendererMain
simple background pages are also cached!
Merge branch '0.4'

Conflicts:
	hoodle-render/src/Graphics/Hoodle/Render.hs
	hoodle-render/src/Graphics/Hoodle/Render/Background.hs