This fixed a problem with libpixman getting the depth wrong for some
16bpp image formats but still render things correctly. Using
surface->depth instead of image->depth for the ximage was a quick way to
avoid this problem.

It's not the appropriate fix and should probably not go upstream.
libpixmap should be fixed if it hasn't already.

================================================================================
--- src/cairo-xlib-surface.c
+++ src/cairo-xlib-surface.c
@@ -796,7 +796,7 @@
     ximage.bitmap_unit = 32;	/* always for libpixman */
     ximage.bitmap_bit_order = native_byte_order;
     ximage.bitmap_pad = 32;	/* always for libpixman */
-    ximage.depth = image->depth;
+    ximage.depth = surface->depth;
     ximage.bytes_per_line = image->stride;
     ximage.bits_per_pixel = bpp;
     ximage.red_mask = red;