Add color filters to the layers display options.

[Merle Moqueur <corvus.lusor@…>]

Hello all,
For now, the available options in the display of layers are opacity, colorfulness, gamma, sharpness.
Let's imagine two situations:

• Top layer has a white background (in France we have the Cadastre, which is a public black, white and yellow vector map of the parcels), and lower layer is a satellite view.
• Top layer is an altitude map in grayscale (for example LIDAR), and lower layer is any color layer.

For now, if we want to use both layers simultaneously it's required to reduce the opacity of the top layer with a muddy, hardly legible result. With filters such as "mutliply", "darken only" and any appropriate filter, the layers would blend together, vastly improving the experience.
Thanks for reading :)