The real problem causing this has been fixed via pencil2d#1729

…as expected (#1667)

* Fix a few cases where fill drag behaviour didn't work as expected

- Filling on layer below when reference layer was all layers, would not allow stroke filling
- Filling with overlay would result in multiple undo operations for one paint operation...
- Click fill with tablet would result in two fill events.

* Fix variable case

* Use pixelformat to check if color is premultiplied when blending occurs

- Use compareColor instead of raw pixel comparison for the ability to fill when the pixel is in average the same.

* Fix linux compilation error

* Fix compiler error caused by inline function definition

* Cleanup

* Fix now unnecessary check for premult

The real problem causing this has been fixed via #1729

* Fix filling was not allowed when fillTo mode was "layer below"

Layer setup:
Bitmap Layer Stroke
Bitmap Layer Fill

1. draw a circle on the stroke layer and slice it up with lines, so you can fill individual parts
2. set fillTo mode to "layer below" and reference to "all layers"
3. fill and drag a color, eg. red.. across the slices
4. Change fillTo mode to "current layer"
5. repeat step 3 with a different color

* Rewrite test cases

* Simplify allowFill function

* Rework drag to fill behavior again

* Fix / simplify allowFill()

* Reference mode should enforce fillTo mode when currentLayer is set.

- Also fixed a few confusing variables

* Fix build error

Co-authored-by: scribblemaniac <scribblemaniac@users.noreply.github.com>
Co-authored-by: Jakob Gahde <j5lx@fmail.co.uk>