Reference: pmRGBCombine

Essentially, the pmRGBCombine function creates a pixmap whose red, green, and blue components are taken from three different pixmaps.

pmRGBCombine(R, G, B) creates a pixmap P such that

 * the red component of pixel (i, j) in P is equal to the red component of (i, j) in R,
 * the green component of pixel (i, j) in P is equal to the green component of (i, j) in G, and
 * the blue component of pixel (i, j) in P is equal to the blue component of (i, j) in B.

The pixmaps R, G, and B need to have the same dimensions, and P will also have these dimensions.