Fix a bug when enabling mirroring and trying the scratchpad

When enabling horizontal and vertical mirroring, painting on the scratchpad
was broken. This was because the scratchpad creates a KisResourceSnapshot
with a 0 image pointer. The snapshot then created a default KisDefaultBounds
object with a 0 image pointer.

That means the KisToolFreehandHelper was told to use an infinite rect
for calculating the mirroring, and iterating over infinity takes a bit of
time.

Fixed by passing the scratchpad's defaultBounds object to the freehand
tool helper, which passes it to the resources snapshot.