Move color function declarations to kcolor.h.
Rearrange resize code.

Resize into a Gif_Image which is mostly taken from the input.
Improved algorithm for scaling down images.
Improved handling of transparency in improved algorithm.
Introduce a scale_context object.
Add kd3_init_build convenience function.
Do not recalculate kd-tree for global colormap during resize.
Fix improved algorithm to work on mixtures of scaling up & down.
Another major scaling change eliminates resize trails.

We can't just consider the input frame; we must consider the region
of the *full unoptimized output image* that overlaps the input frame.
Generate more transparent pixels.
Document kcscreen.
Add --resize-method option.
Clp: Add Clp_vsnprintf and Clp_fprintf.

And use them to print sexy UTF-8 error messages.
Also update buildstring protocol.
Avoid rounding errors by toning down assertions in favor of enforcement.
kc_debug_str can print colors with negative components.
Fix bug with resizing images with transparent background.

0 opaque pixels must always be transparent; luckily,
0 is always <= (xd * yd) / 4.
Add a test to catch that problem.
Make test does make all.
Add a bilinear interpolation resize method ('mix').

And rename other resize methods more consistently.
More correct manual page description.