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New adaptive subpixel supersampling engine

After many hours of painstaking testing and profiling, I'm finally ready
to commit this bad boy.  (And as usual, I'm quite excited about adding a
new multi-tool engine to PD.)

Ever since @audioglider contributed his excellent Glass Tiles tool with
supersampling support, I've wanted to write a universal supersampling
engine for PD, which could then be dropped into all relevant distort and
transform tools.  Unfortunately, supersampling is complicated, and its
performance penalties are extreme.  Since VB already struggles to handle
all the complicated code we throw at it, any supersampling engine must
be highly optimized and as lightweight as possible, while still
providing all the expected benefits.

I believe the new engine achieves this quite well, leveraging the
expected benefits of supersampling with a good mix of VB-specific
shortcuts and optimizations.  Output is much better than a standard
sampling approach (as PD used previously), yet there is no measurable
performance hit for users who don't want or need supersampling's
benefits.

The new engine accepts a user-supplied Quality parameter on a 1-5 scale.
Qualities 1 and 2 do not use supersampling (they only affect sub-pixel
interpolation), while quality levels 3-5 each add a unique supersampling
grid to the output.

After much profiling, I've decided to go with a hard-coded rotated grid
approach.  All point distributions have trade-offs, but rotated grid is
easy to precalcuate, and it provides quite good results at only 4
sampling sites.  For most users, a Quality level of 3 (which mixes
interpolation and a single grid of 4 supersampled pixels) will provide
extremely good results.  Quality levels 4 and 5 add additional offset
grids at varying distances from the origin, for those who need truly
pristine output.

Because supersampling is so intensive, I've made the new engine
adaptive.  Depending on the quality level, a subsection of the total
supersampling grid will be sampled in advance.  If variance is below a
predetermined threshold (currently an estimated diff of 1.5 per RGBA
channel), supersampling is skipped entirely.  This means that
supersampling performance will vary by both effect and by image, with
larger gains coming from tools that expand image data rather than
contract it, and/or images without tons of noise.

My test tool for the new engine was "Add Lens Distortion".  It's a good
test case as it transforms an image with areas where supersampling is
pointless (the center of the lens), and areas where it's extremely
helpful (the exterior of the lens).  Quality levels 1 and 2 provide
roughly identical performance to the old tool, while quality levels 3-5
will roughly double processing time while providing increasingly
accurate output.  The extreme edges of the lens also receive excellent
antialiasing thanks to this new approach.

Over the next few weeks, I'll continue adding supersampling support to
all relevant tools, but I believe the core engine code is pretty much
complete.  Also note that - as always - tool performance inside the IDE
will be incredibly poor if supersampling is active.