gcpBlendColor - Color blending
#include <ggi/gcp.h>
gcp_pixel gcpBlendColor(ggi_visual_t vis,
const gcp_pixel *pix1, const gcp_pixel *pix2,
uint16_t alpha);
gcpBlendColor blends pix2 over pix1 with the translucency alpha, with
pix1 as the background, pix2 the forground. alpha is a 16bit
translucency channel.
gcpBlendColor returns the resulting gcp_pixel(3) value.