Handle non-zero border on viewer container

[oscarlorentzon]

When a non-zero border is used the library does not behave properly because the canvas is shifted and it is not entirely shown because of the fact that offset* values are used. This leads to the following when having top and left borders.

• Only top left of part photo is shown leading to faulty bouncing among other things.
• All DOM rendered items that use percentages like TagComponent render in the wrong place.

Relevant reference:
https://developer.mozilla.org/en-US/docs/Web/API/CSS_Object_Model/Determining_the_dimensions_of_elements

Currently the library does not support non-zero border. Maybe a note about this in the docs is enough for now.

[gyllen]

I think a note and maybe an assert is enough.