inverted brush shape on the left and top edge of the image

[leclerc]

when the brushstroke goes beyond the left and top edge of the image, the trace is inverted, so not all pixels are selected. see the video below to be clearer.

[leclerc]

the second video is to be clearer than a trace concerning the inversion of the shapes

[agriggio]

Properly handle brush strokes that are partially outside the image area

Fixes #36

→ <<cset 74b785671b7f>>

[leclerc]

ultra quick correction !!! thank you !!!