Minor improvements to Film Grain tool

I had intended to convert this to the Gaussian approximation blur, but
because the softening must be very gradual, the full Gaussian function
is required.

So instead I made some minor performance and quality tweaks to the tool.
Remove unneeded external pcMemDC class

This was originally required by the old custom progress bar class, but
since we now use the system progress bar, we can dump it!
Minor code cleanup
New Chroma Blur tool

Useful as a noise removal option, particularly on digital photos taken
with a low-end camera.  Only color data is smoothed; luminance is
preserved.  This allows for subtle color smoothing while retaining all
edges in an image, which nicely aligns with the way humans perceive
color data.