Transparent window *background*

[DLFW]

It seems that I can only control transparency of a complete window, or widgets, but not the window background.

I would like to set the background of a window to any opacity level, but still be able to have widgets fully opaque.

Related to #968, where @lessp stated:

I guess though, that this puts a transparency on everything, not only the "background-window" as you'd expect.

Most modern terminal emulators are good examples. They mostly offer an option to make the background (slightly) transparent but they keep the fonts fully opaque to keep stuff readable. Just making everything transparent is something that I can also achieve with my window compositor, it’s not necessary that the application takes care about that.