fix: Removed assumption of annotation SVG's existence [DC-498]

[Jarbuckle]

fix: Removed assumption of annotation SVG's existence [DC-498]

What

DZI images without associated annotation SVGs were failing to render. Since it is valid for DZI images to not have associated annotation SVGs, removed this assumption to prevent issues during rendering.

How

• Replaced compose (which was always created via buildCompositor and assumed an SVG's existence) with a variable render that falls back to putImageData when no SVG is found.

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[lanesawyer]

This fixes the issue, but I think the image check logic should live in buildCompositor instead so that we don't have two ctx.putImageData(image, 0, 0) lines floating around.

Not completely opposed to shipping it as-is though. The logic here isn't too complex. Just nervous that it'll grow as we polish up the viewer and then we'll be duplicating more and more instead of having a single function that does the proper drawings.

[Jarbuckle]

I've made updates in the bkp-client PR first, @lanesawyer -- could you take a look over there? If it's good there, I'll make updates on this side.

[lanesawyer]

Looks good after merge!