This patch should resolve an issue where the popupmenu for a combobox
could become 'stranded' if the plugin window was moved while the box was
open.

This behaviour was already present on windows, so this patch updates
Linux and macOS window-handling to match.

Previously we were choosing a Visual and Colormap on initialisation of the X11 Display which could lead to bugs when opening a window with different visual and colour requirements. This commit makes 16, 24 and 32-bit visuals available and chooses the correct one when opening a window.