x11/cinnamon: optionalise fallback panel

When Cinnamon fails to start, the fallback is x11-wm/metacity +
panel, similar to MATE or GNOME Flashback. In control structure
order, x11/mate-panel, x11/gnome-panel and x11/tint are probed for
the presence, so expose these as OPTIONS and set x11/mate-panel as
default.

While here, restore the old BUILD_DEPENDS line for <EGL/eglmesaext.h>.