sdl12-compat: Stub in SDL_GL_Lock, SDL_GL_Unlock, and SDL_GL_UpdateRects.

SDL Commits
1 
From 5cb990acfef2aa9e2593f44222f0489a1e41e886 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Mon, 1 Mar 2021 21:06:04 -0500
Subject: [PATCH] Stub in SDL_GL_Lock, SDL_GL_Unlock, and SDL_GL_UpdateRects.

---
 src/SDL12_compat.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/SDL12_compat.c b/src/SDL12_compat.c
index 9ca3089..9474575 100644
--- a/src/SDL12_compat.c
+++ b/src/SDL12_compat.c
@@ -4543,6 +4543,23 @@ SDL_OpenAudio(SDL_AudioSpec *want, SDL_AudioSpec *obtained)
     return retval;
 }
 
+/* !!! FIXME: these are just stubs for now, but Sam thinks that maybe these
+were added at Loki for Heavy Gear 2's UI. They just make GL calls. */
+void SDL_GL_Lock()
+{
+    FIXME("write me");
+}
+
+void SDL_GL_UpdateRects(int numrects, SDL_Rect *rects)
+{
+    FIXME("write me");
+}
+
+void SDL_GL_Unlock()
+{
+    FIXME("write me");
+}
+
 
 /* SDL_GL_DisableContext and SDL_GL_EnableContext_Thread are not real SDL 1.2
    APIs, but some idTech4 games shipped with a custom SDL 1.2 build that added