Automatically skip unnecessary conversions / resizes

ResizeFilter, RgbFilter, YuvFilter now automatically skip processing if
the previous filter can resize / convert to the correct size /
colourspace directly.
This removes the need for WantYuv and PrescaleSize in the RenderChain
class.
"CreateFilter" -> "new ShaderFilter"

The "CreateFilter" functions didn't really do anything useful anymore,
and the name "CreateFilter" was ambiguous.