Lasso selections: plumbing for "smooth stroke" capabilities

One of the neat things PhotoDemon is already set up for is rendering
arbitrary paths using cardinal splines instead of line segments.  This
allows for a much more natural curvature of strokes while painting or
drawing, and is a feature that even Photoshop lacks.  (In other editors,
it's sometimes called "smooth stroke", "steady stroke", or "stroke
stabilizing".)

I've now added support for this feature to lasso selections, but I'm not
entirely pleased with its performance just yet.  To really get ideal
results from it, we need to do some manual clean-up of the path data
coming from the mouse, specifically removing redundant data points and
applying some pre-smoothing to the stroke, before handing it off to the
curve renderer.

I don't want to get too off track this early in 6.6's development cycle,
so this commit basically commits the plumbing for smooth stroking lasso
selections, but leaves all the UI elements hidden.  Once I've got some
standardized "path clean-up" functions written, I can drop them into the
selection engine and re-enable these UI elements.