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Expose colour_mix() to backends (and others)

Quite a few backends currently generate colours by multiplying the
foreground colour by a fraction, effectively mixing it with black.  On a
black background, this might be reasonably replaced by mixing the
background colour with white, but that's rather arithmetically fiddly.
Happily, I already have a function for that and just need to expose it.

--- a/devel.but

+++ b/devel.but

@@ -4937,6 +4937,20 @@

 criterion in a dynamic manner without having to write global variables

 in the caller for the compare function to read.

+\S{utils-colour-mix} \cw{colour_mix()}

+

+\c void colour_mix(const float src1[3], const float src2[3], float p,

+\c                 float dst[3]);

+

+This function mixes the colours \c{src1} and \c{src2} in specified

+proportions, producing \c{dst}.  \c{p} is the proportion of \c{src2}

+in the result.  So if \c{p} is \cw{1.0}, \cw{dst} will be the same as

+\c{src2}.  If \c{p} is \cw{0.0}, \cw{dst} will be the same as

+\c{src1}.  And if \{p} is somewhere in between, so will \c{dst} be.

+\c{p} is not restricted to the range \cw{0.0} to \cw{1.0}.  Values

+outside that range will produce extrapolated colours, which may be

+useful for some purposes, but may also produce impossible colours.

+

 \S{utils-game-mkhighlight} \cw{game_mkhighlight()}

 \c void game_mkhighlight(frontend *fe, float *ret,

--- a/misc.c

+++ b/misc.c

@@ -207,8 +207,7 @@

                        (a[2]-b[2]) * (a[2]-b[2]));

 }

-static void colour_mix(const float src1[3], const float src2[3],

-                       float p, float dst[3])

+void colour_mix(const float src1[3], const float src2[3], float p, float dst[3])

 {

     int i;

     for (i = 0; i < 3; i++)

--- a/puzzles.h

+++ b/puzzles.h

@@ -374,6 +374,9 @@

 char *bin2hex(const unsigned char *in, int inlen);

 unsigned char *hex2bin(const char *in, int outlen);

+/* Mixes two colours in specified proportions. */

+void colour_mix(const float src1[3], const float src2[3], float p,

+                float dst[3]);

 /* Sets (and possibly dims) background from frontend default colour,

  * and auto-generates highlight and lowlight colours too. */

 void game_mkhighlight(frontend *fe, float *ret,