Compositing on image/solid color

[hpjansson]

Add the ability to composite the scaled output on an existing image buffer or a solid color. Allow origin outside the target image (i.e. negative top-left offsets) with clipping as appropriate and subpixel placement.

[hpjansson]

When compositing on a solid color, we need to be able to generate a destination image that's bigger than the source.

This is especially important for the width dimension, since there's no efficient way for the user to manually add columns to an image. Adding rows above/below is much easier; just allocate a bigger buffer, clear the area above/below, and offset the dest pointer. You could do something similar for columns using rowstride, but clearing vertical strips is fiddly and cache-inefficient.

The over-color API should probably be similar to the over-image one; we need to specify dimensions and offset, allow for clipping and so on. The main difference would be that over-color should take a pointer to a single pixel representing the color.

We could take this further and support the general case, where "solid color" would just be a tiled one-pixel image, but that's probably out of scope for now.