Replace hand-crafted pixel format conversion routines with Pixman calls

The following discussion from !458 should be addressed:

• @pq started a discussion: (+2 comments)

  Oh my, I didn't even realize these were duplicated already.

  They should be replaced with pixman_image_composite32() calls, but that's another story. Just leave them here.

Instead of writing our own functions to copy the screenshot to the client buffer, it is better to use pixman functions instead.