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

README.md

@@ -116,6 +116,13 @@ information on what to include when reporting a bug.
 
 ## Changelog
 
+ - Added `GLFW_PLATFORM` init hint for runtime platform selection (#1958)
+ - Added `GLFW_ANY_PLATFORM`, `GLFW_PLATFORM_WIN32`, `GLFW_PLATFORM_COCOA`,
+   `GLFW_PLATFORM_WAYLAND`, `GLFW_PLATFORM_X11` and `GLFW_PLATFORM_NULL` symbols to
+   specify the desired platform (#1958)
+ - Added `glfwGetPlatform` function to query what platform was selected (#1655,#1958)
+ - Added `glfwPlatformSupported` function to query if a platform is supported
+   (#1655,#1958)
  - Added `glfwInitAllocator` for setting a custom memory allocator (#544,#1628,#1947)
  - Added `GLFWallocator` struct and `GLFWallocatefun`, `GLFWreallocatefun` and
    `GLFWdeallocatefun` types (#544,#1628,#1947)
@@ -126,12 +133,16 @@ information on what to include when reporting a bug.
  - Added `GLFW_POINTING_HAND_CURSOR` alias for `GLFW_HAND_CURSOR` (#427)
  - Added `GLFW_MOUSE_PASSTHROUGH` window hint for letting mouse input pass
    through the window (#1236,#1568)
+ - Added `GLFW_PLATFORM_UNAVAILABLE` error for platform detection failures (#1958)
  - Added `GLFW_FEATURE_UNAVAILABLE` error for platform limitations (#1692)
  - Added `GLFW_FEATURE_UNIMPLEMENTED` error for incomplete backends (#1692)
  - Added `GLFW_ANGLE_PLATFORM_TYPE` init hint and `GLFW_ANGLE_PLATFORM_TYPE_*`
    values to select ANGLE backend (#1380)
  - Added `GLFW_X11_XCB_VULKAN_SURFACE` init hint for selecting X11 Vulkan
    surface extension (#1793)
+ - Added `GLFW_BUILD_WIN32` CMake option for enabling Win32 support (#1958)
+ - Added `GLFW_BUILD_COCOA` CMake option for enabling Cocoa support (#1958)
+ - Added `GLFW_BUILD_X11` CMake option for enabling X11 support (#1958)
  - Added `GLFW_LIBRARY_TYPE` CMake variable for overriding the library type
    (#279,#1307,#1497,#1574,#1928)
  - Added `GLFW_PKG_CONFIG_REQUIRES_PRIVATE` and `GLFW_PKG_CONFIG_LIBS_PRIVATE` CMake
@@ -140,6 +151,8 @@ information on what to include when reporting a bug.
  - Made `GLFW_DOUBLEBUFFER` a read-only window attribute
  - Updated the minimum required CMake version to 3.1
  - Disabled tests and examples by default when built as a CMake subdirectory
+ - Renamed `GLFW_USE_WAYLAND` CMake option to `GLFW_BUILD_WAYLAND` (#1958)
+ - Removed `GLFW_USE_OSMESA` CMake option enabling the Null platform (#1958)
  - Removed CMake generated configuration header
  - Bugfix: The CMake config-file package used an absolute path and was not
    relocatable (#1470)
@@ -442,6 +455,7 @@ skills.
  - Eddie Ringle
  - Max Risuhin
  - Jorge Rodriguez
+ - Jari Ronkainen
  - Luca Rood
  - Ed Ropple
  - Aleksey Rybalkin