Avoid upscaling of bird's eye image (rebased onto dev_5_0)



This is the same as gh-2550 but rebased onto dev_5_0.

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As @mtbc mentioned in PR #2504 Insight's bird's eye view is ugly pixellated. The reason for this is, that the image is loaded with 96px (default thumbnail size) and then upscaled to e. g. 196px. This PR removes the upscaling and loads the image with the right size in the first place, because I don't think that it makes a noticeable performance difference, if the image is loaded as 96px instead of 256px (which is the largest possible image size for the bird's eye view).

<b>Test:</b> Open some big images, make sure that the bird's eye view is still working and there is no performance setback.