Wayland: Improve rendering performance

[mark-kendall]

b6e7e18 fixed alpha blending issues when using Wayland but as noted in the commit this is not the most performant solution and we really need to call wl_surface_set_opaque_region to allow the compositor to optimise rendering. The current solution is also not guaranteed to continue working in the future.

This requires:-

• linking to the wayland library (which is not currently required elsewhere)
• retrieving both the current compositor and surface from the QT wayland platform plugin
• update the opaque region for window resizes
• retrieving compositor/surface needs QPlatformNativeInterface defined - which requires Qt private headers

[mark-kendall]

#218 created for underlying wayland compile time support. This ticket will track subsequent enhancements to actually call wl_surface_set_opaque_region