Photoshop-like viewport scrolling

[ktsn]

I'm working on resizable (#19) and zoomable viewport. But I just noticed it is probably needed to implement photoshop-like scrolling UI as well. The normal browser scroller (overflow: auto;) well works on normal documents but does not seems to work on this kind of tool.

For example, if I want to resize the viewport to much large size, it will be immediately blocked because the scroll is limited. I expect the renderer allows to scroll for much large size than viewport size so that I easily resize the viewport more larger.