Added sharpness filter

[rakama]

When tiles are resized, a sharpening filter is applied. The sharpness intensity will vary depending on zoom level. This can be adjusted by changing these lines in overviewer_core/tileset.py:

[495] # Intensity of the sharpness filter when zoomed all the way in, range = [0, 1]
[496] initial_sharpness = 0.4
[497] # Reduction in the filter's intensity for each level zoomed out, range = [0, 1]
[498] decay_rate = 0.5

Sharpness can range from 0 (no effect) to 1 (maximum effect). If initial_sharpness = 0, the code for the sharpening filter is never executed. For each level zoomed out, the intensity is multiplied by decay_rate. If the decay_rate is set to 1, the effect intensity will stay fixed regardless of zoom.

Things that should probably be evaluated before being enabled in master:

• Impact on running time (is it too slow?)
• Subjective measures of quality (is it ugly, good looking, what?)
• Best values for intensity/decay (what is the proper balance?)

[agrif]

(just a quick note: this is being pushed off till it can be integrated into oil, as part of a more general image post-processing step)