Make scaling alpha aware

[spillerrec]

Not keeping alpha into account when scaling can create artifacts, as the otherwise "transparent" colors are included in the interpolation, causing black/white to bleed into the image.

[spillerrec]

It is not quite correct yet, but it is close.
AverageRender was modified to scale the output planes so that we use the alpha generated for each plane. The ImageEx::alpha_plane() is for the full scale image, so it cannot directly be applied to images which has been sub-sampled. This is likely going to pop up again in the future, so we need some way to deal with this. An assert is a good start.

I changed the interface on ImageEx to always require to specify an alpha plane. The following classes didn't have an obvious alpha plane to use, so they were given a null plane:

• EstimatorRender
• ModifiedPlane

They should be corrected. Consider checking if all plane scaling also uses the alpha version as well.