Wayland: Add initial support for libdecor

This is partly based on the implementation of libdecor support in
PR #1693 by @ christianrauch.

Where available, the libdecor library is loaded at init and becomes the
preferred method for window decorations.  On compositors that support
XDG decorations, libdecor in turn uses those.  If not, libdecor has
a plug-in archtecture and may load additional libraries to either use
compositor-specific decorations or draw its own.

If necessary, support for libdecor can be disabled with the
GLFW_WAYLAND_LIBDECOR init hint.  This is mostly in case some part of
the dynamic loading or duplication of header material added here turns
out to cause problems with future versions of libdecor-0.so.0.

Fixes #1639
Closes #1693
Related to #1725

src/init.c

@@ -62,6 +62,9 @@ static _GLFWinitconfig _glfwInitHints =
     {
         GLFW_TRUE,  // X11 XCB Vulkan surface
     },
+    {
+        GLFW_WAYLAND_PREFER_LIBDECOR // Wayland libdecor mode
+    },
 };
 
 // The allocation function used when no custom allocator is set
@@ -479,6 +482,9 @@ GLFWAPI void glfwInitHint(int hint, int value)
         case GLFW_X11_XCB_VULKAN_SURFACE:
             _glfwInitHints.x11.xcbVulkanSurface = value;
             return;
+        case GLFW_WAYLAND_LIBDECOR:
+            _glfwInitHints.wl.libdecorMode = value;
+            return;
     }
 
     _glfwInputError(GLFW_INVALID_ENUM,