demos: Get rid of some unused includes.
These were used for demo_load_shader(), but that was removed some time
ago in commit aa5d48eec4987c4c1cb55571b536cd485c25740b.
demos: Move the vkd3d parts of demo_xcb.h into a separate header.
So that they can be used for e.g. Wayland or macOS support. Or Windows,
if we're so inclined.