GHOST: Add environment variable for enabling or disabling vsync

This PR proposes to add an environment variable for forcing vsync to
be on or off. My primary use case was to disable vsync for forcing
viewport rendering performance tests not to be capped at the display
refresh rate when #142984 is used for removing animation frame rate
limits.

I initially added the environment variable "GHOST_VSYNC_OFF", but
found "GHOST_VSYNC=0/1" to be more easily understandable.

Usage:
- GHOST_VSYNC=0 => Vsync is forced off
- GHOST_VSYNC=1 => Vsync is forced on

Pull Request: https://projects.blender.org/blender/blender/pulls/143049

source/creator/creator_args.cc

@@ -866,6 +866,10 @@ static void print_help(bArgs *ba, bool all)
   PRINT("  $BLENDER_CUSTOM_SPLASH     Full path to an image that replaces the splash screen.\n");
   PRINT(
       "  $BLENDER_CUSTOM_SPLASH_BANNER Full path to an image to overlay on the splash screen.\n");
+  PRINT(
+      "  $GHOST_VSYNC Whether to disable (0) or enable (1) vsync. Otherwise left to the GHOST "
+      "backend. For the OpenGL backend, the passed value is used as the swap interval "
+      "(https://www.khronos.org/opengl/wiki/Swap_Interval).\n");
 
   if (defs.with_opencolorio) {
     PRINT("  $OCIO                      Path to override the OpenColorIO configuration file.\n");