corrected sharpening for thumbs and made gammacorrection optional

added a calculation / adjustment for a USM-factor that indicates how
much an image should be sharpened according to a resizing scalevalue and
the absolute target dimensions

minor fix: make imSharpen aware of the option 'useUSM'

change setDefaultGamma to accept a wider range of values

make the usage of GammaCorrection optional, - Sending a -1 to the
setDefaultGamma() disables it.  = ('defaultGamma' as image option per
template or in $config->imageSizerOptions['defaultGamma'] sitewide)