Revert "- let gl_draw_sync default to false."

This reverts commit 616f84209b.

Now I remember why this is not active: This is causing some problems with interpolated plane heights. Since there is no synchronization with the vertex buffer, the next frame will reset all plane coordinates before the GPU can use them. This will require some rethinking about when to update the buffer - it cannot be done in the render data setup loop.

src/gl/system/gl_framebuffer.cpp

@@ -164,7 +164,8 @@ void OpenGLFrameBuffer::InitializeState()
 //
 //==========================================================================
 
-CVAR(Bool, gl_draw_sync, false, CVAR_ARCHIVE|CVAR_GLOBALCONFIG)
+// Testing only for now. 
+CVAR(Bool, gl_draw_sync, true, 0) //false, CVAR_ARCHIVE|CVAR_GLOBALCONFIG)
 
 void OpenGLFrameBuffer::Update()
 {