glwaylanddisplay: Expects structure field "display" while waylandsink uses "handle"

This apparently has been broken since few years. When extracting the display from the GstContext structure, gstgl expect the field "display" and waylandsink (through the internal wayland library) expects "handle".

Here's my proposal. It would be confusing if we'd pick one or another, as it will possibly break half the apps. So because it has been broken for quite sometimes, I'd document the one I prefer, "display", but in the code, I'd support both. That consist of:

• Checking for both in glimagesink
• Setting/checking both in gstwayland
• Fixing any example to use "display"

I would also like to add a warning if no display was found in the GstContext. This can only be a programming error, but right now glimagesink silently ignores it. Later on it will try to use foreigh surfaces with its own display, which will break in very strange way.