During an effect preview, shift the preview ratio burden to prepImageData

Previously, functions that operate on a radius (like blur) had to
perform their own math when rendering a preview, to make it so that the
previewed blur strength was correctly representative of the final blur
strength.

This was a foolish design decision.  prepImageData is perfectly capable
of calculating this itself, and it now does so.  Now, calling functions
can simply borrow the calculated data as necessary.

Also, greatly improved comments in this module with this commit.
Fix Glass filter to accurately render preview strength

Previously, the preview and final image looked quite different.
Fix comments
Fix Ripple filter preview; it now approximates the final image much more accurately
Fix Waves filter preview; it now approximates the final image much more accurately
Simplify multiple tools to take advantage of the new preview modifier calculation technique

Less code all around!  Yay!