This refactor includes:
- In Lua now we can clone a custom brush with Brush(Image) and the new
brush doesn't share the image with the original one (added a new test
for this).
- Avoid creating extra images when it's not needed using
Brush::cloneWithExistingImages() (we can inject existing images in
the brush itself).
- Delete Brush-copy contructor & assign operator to use
Brush::clone() functions instead (which are more explicit).
- Some code from 12d8135264 (#4023)
reverted to avoid recreating brushes on left-click or in the brush
preview, i.e. moving the mouse (#4013 refers only to right-click, so
only on right-click we have to adjust the custom brush).
Before this fix to change the main color of the image brush,
it was necessary to choose a new color from the palette.
The secondary color can now also be used.
Also added some tests for image brushes.
