Switch versioning scheme to be the same as GLib and Flatpak

For stable releases, this gives us the ability to make bugfix-only point
releases such as 2.24.1 if we want to, and distinguish between them
programmatically. For example, this ability could have been useful after
2.0.16 to fix Xwayland regressions, and after 2.0.18 to fix event loop
regressions.

For development releases, this gives us the ability to make multiple
prereleases during the same feature cycle, and distinguish between them
programmatically. For example, this would have been useful during 2.0.22
development, which went through three prereleases before reaching the
final release.

Signed-off-by: Simon McVittie <smcv@collabora.com>

WhatsNew.txt

@@ -2,10 +2,22 @@
 This is a list of major changes in SDL's version history.
 
 ---------------------------------------------------------------------------
-2.0.24:
+2.24.0:
 ---------------------------------------------------------------------------
 
 General:
+* New version numbering scheme, similar to GLib and Flatpak.
+    * An even number in the minor version (second component) indicates
+      a production-ready stable release such as 2.24.0, which would have
+      been 2.0.24 under the old system.
+      * The patchlevel (micro version, third component) indicates a
+        bugfix-only update: for example, 2.24.1 would be a bugfix-only
+        release to fix bugs in 2.24.0, without adding new features.
+    * An odd number in the minor version indicates a prerelease such
+      as 2.23.0. Stable distributions should not use these prereleases.
+      * The patchlevel indicates successive prereleases, for example
+        2.23.1 and 2.23.2 would be prereleases during development of
+        the SDL 2.24.0 stable release.
 * Added SDL_bsearch() to the stdlib routines
 * Added functions to get the platform dependent name for a joystick or game controller:
     * SDL_JoystickPathForIndex()