Add "multiply" blendmode option to custom overlay layer

[momentmal]

The current BRouter-web hillshade implementation lacks resolution (2), and nowadays there are free offers like basemap.de Hillshade Layer for Germany which include LIDAR-Level Details (3).
For Custom Layers like these, a blend mode, especially multiply, would be hugely benefitial to improve map readability (5) - as adding a 50% transparent layer washes out the map below (4).

The multiply option graphical element could be added in multiple ways, for example:

A: a checkbox beside every custom overlay layer
B: in the add / edit custom layer page as a 4th column entry with a checkbox
C: the third column in the add / edit custom layer page could contain dropdown elements for each layer to chose between base layer, overlay layer or multiplied overlay layer

"But not sure yet if this would be useful in addition to an opacity slider."
Originally posted by @nrenner in #213 (comment)