measureBasedView: fix imageZoom levels with two viewers

[bwbohl]

When MeasureBasedView adds an additional HorizontalMeasureViewer it resizes the viewers to a proportional width, depending on the width of the measure group to be displayed in each viewer compared to the total width of all viewers. This leads to existing viewers' zoom level mismatch.

[bwbohl]

this might lead to a general reconsideration of how window space is spread across multiple parts, especially when considering multi-staff system of, e.g. piano part notation.

[roewenstrunk]

• The image viewers get their widths here: https://github.com/Edirom/Edirom-Online/blob/93828fd9375aa77c9b96e6c6b7690fc4ca4884c7/app/view/window/source/MeasureBasedView.js#L586
• The rectangle to show is specified here: https://github.com/Edirom/Edirom-Online/blob/93828fd9375aa77c9b96e6c6b7690fc4ca4884c7/app/view/window/source/MeasureBasedView.js#L616
• The OpenSeaDragonViewer gets the rectangle here: https://github.com/Edirom/Edirom-Online/blob/93828fd9375aa77c9b96e6c6b7690fc4ca4884c7/app/view/window/image/OpenSeaDragonViewer.js#L146
• The OpenSeaDragonViewer calculates the zoom level for the rectangle here: https://github.com/Edirom/Edirom-Online/blob/93828fd9375aa77c9b96e6c6b7690fc4ca4884c7/app/view/window/image/OpenSeaDragonViewer.js#L158

It looks like the viewer is calculating with wrong dimensions when calculating the zoom for the passed rectangle. This needs to be checked.