Fix various typos

Found via `codespell -q 3 -S ./ltmain.sh,./scripts/libtool.m4 -L ake,ans,ba,bloc,crashers,doed,inout,lengthh,maked,paeth,parm,parms,redy,unx`

contrib/libtests/makepng.c

@@ -1876,7 +1876,7 @@ main(int argc, char **argv)
          }
    }
 
-   /* small and colors are incomparible (will probably crash if both are used at
+   /* small and colors are incompatible (will probably crash if both are used at
     * the same time!)
     */
    if (small && colors[0] != 0)

contrib/libtests/pngvalid.c

@@ -7643,7 +7643,7 @@ image_transform_png_set_rgb_to_gray_ini(const image_transform *this,
 
    else
    {
-      /* The default (built in) coeffcients, as above: */
+      /* The default (built in) coefficients, as above: */
 #     if PNG_LIBPNG_VER < 10700
          data.red_coefficient = 6968 / 32768.;
          data.green_coefficient = 23434 / 32768.;
@@ -11768,7 +11768,7 @@ int main(int argc, char **argv)
 
    /* The following allows results to pass if they correspond to anything in the
     * transformed range [input-.5,input+.5]; this is is required because of the
-    * way libpng treates the 16_TO_8 flag when building the gamma tables in
+    * way libpng treats the 16_TO_8 flag when building the gamma tables in
     * releases up to 1.6.0.
     *
     * TODO: review this

contrib/libtests/timepng.c

@@ -527,7 +527,7 @@ int main(int argc, char **argv)
       }
    }
 
-   else if (nfiles == 0) /* Read from stdin withoout --dissemble */
+   else if (nfiles == 0) /* Read from stdin without --dissemble */
    {
       char filename[FILENAME_MAX+1];