Optimize alpha counting in AverageRender

[spillerrec]

Half of the time is spend on counting how many images affects a specific pixel, and how much. When all the images does not containing a alpha channel, we could optimize this.

We could optimize for just the vertical alignment case, as this is 95% of the cases, or we could try to generalize it to both directions. For that we could have a stack for each line which marks when, and how much the amount of images changes.
For now I propose we just optimize it for the vertical case, which just needs a count for each line.