• added to_rgb16 to produce a 16-bit/sample version of an image

• improve freetype 1.x text output efficiency

Bug fixes:

• search another place for rgb.txt, and check all the places Imager::Color checks when deciding whether to skip testing it

• use a convolution kernel size based on the stddev rather than a fixed size when performing a gaussian blur

• document the difference() method's mindist parameter, and debug it.

• put the Imager release number in the Inline::C generated code to regenerate Inline code when a new release of Imager is installed.

• fix rendering on alpha channel images for the FreeType 1.x driver.

• fix rendering on alpha channel images for the T1lib driver.

• reworked library probing, we can now set more than one probe function for a library. Disabled the default (non-freetype-config) library probe and added an extra probe function that searches for both ft2build.h and whatever it includes, and adds -I as needed. Hopefully this will fix build problems like this cpan testers failure