Improve image depth handling

Improve image depth handling

Emacs defaults to a 32-bit TrueColor visual, but if that happens
on a display which defaults to 16-bit TrueColor, yet happens to
have 32-bit color, and doesn't have the X Render Extension, an
error will occur in x_composite_image as libXpm will load
pixmaps of depth 16 instead of depth 32.

* src/image.c (x_create_x_image_and_pixmap): Explictly specify
display depth.
(x_create_xrender_picture):
(xpm_load):
(gs_load): Use dpyinfo->n_planes instead of
DefaultDepthOfScreen.