rgl: how to produce higher resolution snapshots?

For a book, I want to show two rgl images side-by-side, as in the image below. But the publisher wants images with at least 300 dpi resolution.

I created this using the code below to produce two separate images with rgl.snapshot() and then combined them into a single image with some other software.

• How can I amend the code below to save each image with higher resolution? The images I get are ~ 256 x 256 pixels, I think with 72 dpi resolution.

• Is there a different device driver I could use with rgl.snapshot?

• Is there some way to compose these into a single image using R?

    data(randu)

    # Figure 9.15 - 3D plots
    library(rgl)
    with(randu, plot3d(x, y, z, size=6, axes=FALSE, 
                       xlab = "", ylab = "", zlab = ""))
    box3d(col="gray")

    # left panel                   
    rgl.snapshot("randu0.png")
    # right panel                   
    rgl.viewpoint(theta = -3.8, phi = 3.8, fov = 0, zoom = 0.7)
    rgl.snapshot("randu.png")