add ability to write blitbuffers to PAM files
Merge remote-tracking branch 'upstream/master' into blitbuffer-netpbm
drop 16bit grayscale blitbuffer implementation, replace by 16bit RGB

Kobos (and in the according mode, other readers probably, too) do not
have a 16bit grayscale framebuffer mode but rather an implementation
of 16bit RGB framebuffers. So we implement that now.
oversight old Color16 reference
bugfix for RGB32 to RGB16 conversion
test for RGB16 instead of 16bpp grayscale
Merge pull request #109 from hwhw/master

Blitbuffer fix: drop 16bpp grayscale, introduce 16bpp RGB