harmonic: Add boilerplate for mouse point transformation..

Rename grid to harmonicGrid.
harmonic: Removal of redundant calculations.

Rename colWidth and rowHeight to cellWidth/Height.

Unnecessary to have x and y parameters in Harmonic.convert2d function.
harmonic: Recalculate weights only when vertices are dirty or parameters have changed.
harmonic: Update placeholder interpolate2d coordinate space.

The transformed point is relative to the coordinate space defined by
the vertices’ AABB.