Improve script performance by generating downscaled images

[Glodenox]

Zooming in on an image overlay goes quite slow for big images. By generating a smaller image in advance for other zoom levels, it should be possible to increase the performance of the script.

[Glodenox]

To investigate:

• Scaling images on the fly whenever someone zooms in or out
• Scaling images on the fly, but also pre-scaling the two nearest zoom levels and using them whenever someone zooms in or out
• Pre-scaling a couple of images and switching out those at certain zoom intervals
• Replacing the image used in the map layer with a canvas layer and scaling and cropping the image in that layer
• Generating image tiles locally on the fly with canvas (might require less recalculations?)

But also keep in mind that maybe browsers are just better at handling big images by themselves after all.