Start tuxpaint, select the arrow brush and the line tool, draw from the
bottom to the up various times.
I get the brush not right distributed, some are close, some are
separate. The cat brush does something similar, the paint tool with
those brushes too.
This occurs as well with non directional brushes, but is better seen in wide spaced ones.
This problem seems to occur because y axis is reversed to allow counting up in brush_draw() and instead, blit_brush() counts continually up to brush spacing
The attached file is a path I propose, it removes the reverse of y axis and instead adds a counting down.
Double check it, I'm not a programmer.
Hope this helps.