Layer: Backdrop Mode

[MorrowShore]

This feature involves a "backdrop" mode layer.

This layer will just be a single color, and changing it would not cause lag like bucket painting would cause.

Its icon would also need to be changed to reflect it being backdrop.

It would go here:

[askmeaboutlo0m]

This is what other programs call a "fill layer". But I don't think it's relevant for Drawpile, you can set a canvas background already, we don't need a layer type for it.

[MorrowShore]

This feature was originally meant to alleviate the slow action of bucket filling an empty layer to serve as backdrop.

However, bucket tool can be optimized (#1414), which will also alleviate this issue.

Will leave this issue open just in case.