[tomo] Fix h5web's SelectionRect performance issue on chrome

This MR workaround two issues with chrome when zooming in with a SelectionRect with dashed strokes:

• In this case, the more you zoom-in (and so the more the SelectionRect is big around the canvas), the worst performances are...Firefox is less affected by this. Also it is only with dashed strokes.
• Also when zooming-in a lot on a line, it's rendering becomes weird and the line might even disappear...

This MR mitigates both issues by clamping the SVG rect to a rect slightly bigger than the canvas size (so the stroke is not visible when outside.

I only applied this fix to SelectionRect in componenets/tomo, I did not changed components/plotting/RoiSelectionTool. The up-coming version of h5web will make some change, and both SvgScene and SelectionRect will have to be updated, but this is fixing the BM05 issue in the meantime.

Thanks @bocciare for the help with the investigation!

attn @loic.huder