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

This was adapted to 3.3-stable from
fbdb53b9ca.

docs/compat.dox

@@ -122,6 +122,14 @@ wayland-protocols 1.6, and mandatory at build time.  If the running compositor
 does not support this protocol, the screensaver may start even for full screen
 windows.
 
+GLFW uses the [libdecor library](https://gitlab.freedesktop.org/libdecor/libdecor)
+for window decorations, where available.  This in turn provides good quality
+client-side decorations (drawn by the application) on desktop systems that do
+not support server-side decorations (drawn by the window manager).  On systems
+that do not provide either libdecor or xdg-decoration, very basic window
+decorations are provided.  These do not include the window title or any caption
+buttons.
+
 GLFW uses the [xdg-decoration
 protocol](https://cgit.freedesktop.org/wayland/wayland-protocols/tree/unstable/xdg-decoration/xdg-decoration-unstable-v1.xml)
 to request decorations to be drawn around its windows.  This protocol is part

docs/intro.dox

@@ -102,6 +102,30 @@ a nib or manually, when the first window is created, which is when AppKit is
 initialized.  Set this with @ref glfwInitHint.
 
 
+@subsubsection init_hints_wayland Wayland specific init hints
+
+@anchor GLFW_WAYLAND_LIBDECOR_hint
+__GLFW_WAYLAND_LIBDECOR__ specifies whether to use
+[libdecor](https://gitlab.freedesktop.org/libdecor/libdecor) for window
+decorations where available.  Possible values are `GLFW_WAYLAND_PREFER_LIBDECOR`
+and `GLFW_WAYLAND_DISABLE_LIBDECOR`.  This is ignored on other platforms.
+
+@note This init hint was added in 3.3.9 and is not present in earlier patch releases.  It
+is safe to attempt to set this hint on earlier versions of GLFW 3.3 but it will emit
+a harmless @ref GLFW_INVALID_ENUM error.  If you need to avoid causing any errors, you can
+check the library version first with @ref glfwGetVersion.
+
+@note To set this hint while also building against earlier versions of GLFW 3.3, you can
+use the numerical constants directly.
+
+@note @code
+int minor, patch;
+glfwGetVersion(NULL, &minor, &patch);
+if (minor > 3 || (minor == 3 && patch >= 9))
+    glfwInitHint(0x00053001 /*GLFW_WAYLAND_LIBDECOR*/, 0x00038002 /*GLFW_WAYLAND_DISABLE_LIBDECOR*/);
+@endcode
+
+
 @subsubsection init_hints_values Supported and default values
 
 Initialization hint             | Default value | Supported values
@@ -109,6 +133,7 @@ Initialization hint             | Default value | Supported values
 @ref GLFW_JOYSTICK_HAT_BUTTONS  | `GLFW_TRUE`   | `GLFW_TRUE` or `GLFW_FALSE`
 @ref GLFW_COCOA_CHDIR_RESOURCES | `GLFW_TRUE`   | `GLFW_TRUE` or `GLFW_FALSE`
 @ref GLFW_COCOA_MENUBAR         | `GLFW_TRUE`   | `GLFW_TRUE` or `GLFW_FALSE`
+@ref GLFW_WAYLAND_LIBDECOR      | `GLFW_WAYLAND_PREFER_LIBDECOR`  | `GLFW_WAYLAND_PREFER_LIBDECOR` or `GLFW_WAYLAND_DISABLE_LIBDECOR`
 
 
 @subsection intro_init_terminate Terminating GLFW

docs/news.dox

@@ -37,6 +37,16 @@ SDK](https://vulkan.lunarg.com/).
 For more information see @ref vulkan_guide.
 
 
+@subsubsection wayland_libdecor_33 Wayland libdecor decorations
+
+GLFW now supports improved fallback window decorations via
+[libdecor](https://gitlab.freedesktop.org/libdecor/libdecor).
+
+Support for libdecor can be toggled before GLFW is initialized with the
+[GLFW_WAYLAND_LIBDECOR](@ref GLFW_WAYLAND_LIBDECOR_hint) init hint.  It is
+enabled by default.
+
+
 @subsubsection content_scale_33 Content scale queries for DPI-aware rendering
 
 GLFW now provides content scales for windows and monitors, i.e. the ratio
@@ -443,6 +453,9 @@ Use Wayland or X11 instead.
  - @ref GLFWwindowmaximizefun
  - @ref GLFWwindowcontentscalefun
  - @ref GLFWgamepadstate
+ - @ref GLFW_WAYLAND_LIBDECOR
+ - @ref GLFW_WAYLAND_PREFER_LIBDECOR
+ - @ref GLFW_WAYLAND_DISABLE_LIBDECOR
 
 
 @subsubsection constants_33 New constants in version 3.3