[client] displayserver: add new earlyInit to the interface

Some platforms such as Wayland need to set environment vairables before
SDL is initialized, as such this change detects the display server
before SDL has started and calls the new `earlyInit` method providing
the implementation an opportunity to set things up.

client/displayservers/SDL/sdl.c

@@ -33,6 +33,11 @@ struct SDLDSState
 
 static struct SDLDSState sdl;
 
+static bool sdlEarlyInit(void)
+{
+  return true;
+}
+
 static void sdlInit(SDL_SysWMinfo * info)
 {
   memset(&sdl, 0, sizeof(sdl));
@@ -143,6 +148,7 @@ static void sdlWarpMouse(int x, int y, bool exiting)
 struct LG_DisplayServerOps LGDS_SDL =
 {
   .subsystem      = SDL_SYSWM_UNKNOWN,
+  .earlyInit      = sdlEarlyInit,
   .init           = sdlInit,
   .startup        = sdlStartup,
   .shutdown       = sdlShutdown,