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

.github/workflows/build.yml

@@ -31,8 +31,8 @@ jobs:
             - name: Build shared library
               run: cmake --build build-shared --parallel
 
-    build-linux-wayland-clang:
-        name: Wayland (Linux, Clang)
+    build-linux-full-clang:
+        name: X11+Wayland (Linux, Clang)
         runs-on: ubuntu-latest
         env:
             CC: clang
@@ -42,38 +42,15 @@ jobs:
             - name: Install dependencies
               run: |
                   sudo apt update
-                  sudo apt install wayland-protocols libwayland-dev libxkbcommon-dev
+                  sudo apt install libxrandr-dev libxinerama-dev libxcursor-dev libxi-dev libxext-dev wayland-protocols libwayland-dev libxkbcommon-dev
 
             - name: Configure static library
-              run: cmake -S . -B build-static -D GLFW_USE_WAYLAND=ON
+              run: cmake -S . -B build-static -D GLFW_BUILD_WAYLAND=ON
             - name: Build static library
               run: cmake --build build-static --parallel
 
             - name: Configure shared library
-              run: cmake -S . -B build-shared -D GLFW_USE_WAYLAND=ON -D BUILD_SHARED_LIBS=ON
-            - name: Build shared library
-              run: cmake --build build-shared --parallel
-
-    build-linux-null-clang:
-        name: Null (Linux, Clang)
-        runs-on: ubuntu-latest
-        env:
-            CC: clang
-            CFLAGS: -Werror
-        steps:
-            - uses: actions/checkout@v2
-            - name: Install dependencies
-              run: |
-                  sudo apt update
-                  sudo apt install libosmesa6-dev
-
-            - name: Configure static library
-              run: cmake -S . -B build-static -D GLFW_USE_OSMESA=ON
-            - name: Build static library
-              run: cmake --build build-static --parallel
-
-            - name: Configure shared library
-              run: cmake -S . -B build-shared -D GLFW_USE_OSMESA=ON -D BUILD_SHARED_LIBS=ON
+              run: cmake -S . -B build-shared -D GLFW_BUILD_WAYLAND=ON -D BUILD_SHARED_LIBS=ON
             - name: Build shared library
               run: cmake --build build-shared --parallel