Size inits as (1,1) and logical size is applied early

[almarklein]

This fixes two problems that seem to occur:

• The canvas was initialized with size (0, 0).
• For some backends, setting the logical size has no immediate effect (most notably the jupyter backend).

This PR:

• Initializes the size (before it knows the backend's actual size) to (1, 1).
• When the logical size is set, it is applied immediately, and possibly adjusted when the backend updates its new physical size. In the time between that, the total_pixel_ratio is adjusted, effectively creating a temporary zoom effect. In most cases you'll never observe this, because the new physical size likely resolves before the next draw.
• Refactors the size-info dict to be more stand-alone.
• Tests.
• Adjust docs.