Add runtime platform selection

This adds compile-time support for multiple platforms and runtime
detection of them.  Window system related platform functions are now
called from shared code via the function pointer struct _GLFWplatform.

The timer, thread and module loading platform functions are still called
directly by name and the implementation chosen at link-time.  These
functions are the same for any backend on a given OS, including the Null
backend.

The platforms are now enabled via CMake dependent options following the
GLFW_BUILD_<platform> pattern instead of a mix of automagic and ad-hoc
option names.  There is no longer any option for the Null backend as it
is now always enabled.

Much of the struct stitching work in platform.h was based on an earlier
experimental branch for runtime platform selection by @ronchaine.

Every platform function related to windows, contexts, monitors, input,
event processing and Vulkan have been renamed so that multiple sets of
them can exist without colliding.  Calls to these are now routed through
the _glfw.platform struct member.  These changes makes up most of this
commit.

For Wayland and X11 the client library loading and display creation is
used to detect a running compositor/server.  The XDG_SESSION_TYPE
environment variable is ignored for now, as X11 is still by far the more
complete implementation.

Closes #1655
Closes #1958

docs/news.dox

@@ -9,6 +9,14 @@
 
 @subsection features_34 New features in version 3.4
 
+@subsubsection runtime_platform_34 Runtime platform selection
+
+GLFW now supports being compiled for multiple backends and selecting between
+them at runtime with the @ref GLFW_PLATFORM init hint.  After initialization the
+selected platform can be queried with @ref glfwGetPlatform.  You can check if
+support for a given platform is compiled in with @ref glfwPlatformSupported.
+
+
 @subsubsection standard_cursors_34 More standard cursors
 
 GLFW now provides the standard cursor shapes @ref GLFW_RESIZE_NWSE_CURSOR and
@@ -63,7 +71,23 @@ Alt-and-then-Space shortcuts.  This may be useful for more GUI-oriented
 applications.
 
 
-@subsection caveats_34 Caveats for version 3.4
+@subsection caveats Caveats for version 3.4
+
+@subsubsection native_34 Multiple sets of native access functions
+
+Because GLFW now supports runtime selection of platform (window system), a library binary
+may export native access functions for multiple platforms.  Starting with version 3.4 you
+must not assume that GLFW is running on a platform just because it exports native access
+functions for it.  After initialization you can query the selected platform with @ref
+glfwGetPlatform.
+
+
+@subsubsection version_string_34 Version string format has been changed
+
+Because GLFW now supports runtime selection of platform (window system), the version
+string returned by @ref glfwGetVersionString has been expanded.  It now contains the names
+of all APIs for all the platforms that the library binary supports.
+
 
 @subsubsection joysticks_34 Joystick support is initialized on demand
 
@@ -118,6 +142,18 @@ GLFW_TRANSPARENT_FRAMEBUFFER on Windows 7 if DWM transparency is off
 
 @subsection removals_34 Removals in 3.4
 
+@subsubsection osmesa_option_34 GLFW_USE_OSMESA CMake option has been removed
+
+This option was used to compile GLFW for the Null platform.  The Null platform is now
+always supported.  To produce a library binary that only supports this platform, the way
+this CMake option used to do, you will instead need to disable the default platform for
+the target OS.  This means setting the @ref GLFW_BUILD_WIN32, @ref GLFW_BUILD_COCOA or
+@ref GLFW_BUILD_X11 CMake option to false.
+
+You can set all of them to false and the ones that don't apply for the target OS will be
+ignored.
+
+
 @subsubsection wl_shell_34 Support for the wl_shell protocol has been removed
 
 Support for the wl_shell protocol has been removed and GLFW now only supports
@@ -130,6 +166,8 @@ then GLFW will fail to initialize.
 @subsubsection functions_34 New functions in version 3.4
 
  - @ref glfwInitAllocator
+ - @ref glfwGetPlatform
+ - @ref glfwPlatformSupported
 
 
 @subsubsection types_34 New types in version 3.4
@@ -142,6 +180,14 @@ then GLFW will fail to initialize.
 
 @subsubsection constants_34 New constants in version 3.4
 
+ - @ref GLFW_PLATFORM
+ - @ref GLFW_ANY_PLATFORM
+ - @ref GLFW_PLATFORM_WIN32
+ - @ref GLFW_PLATFORM_COCOA
+ - @ref GLFW_PLATFORM_WAYLAND
+ - @ref GLFW_PLATFORM_X11
+ - @ref GLFW_PLATFORM_NULL
+ - @ref GLFW_PLATFORM_UNAVAILABLE
  - @ref GLFW_POINTING_HAND_CURSOR
  - @ref GLFW_RESIZE_EW_CURSOR
  - @ref GLFW_RESIZE_NS_CURSOR