Fix videoMap with r_smp 1

It was broken due to a stray call to R_SyncRenderThread in a backend
function.

Fixes DaemonEngine#1263.

src/engine/renderer/tr_backend.cpp

@@ -4994,8 +4994,6 @@ RENDER BACK END THREAD FUNCTIONS
 
 void RE_UploadCinematic( int cols, int rows, const byte *data, int client, bool dirty )
 {
-	R_SyncRenderThread();
-
 	GL_Bind( tr.cinematicImage[ client ] );
 
 	// if the scratchImage isn't in the format we want, specify it as a new texture

src/engine/renderer/tr_cmds.cpp

@@ -201,6 +201,8 @@ OpenGL calls until R_IssueRenderCommands is called.
 */
 void R_SyncRenderThread()
 {
+	ASSERT( Sys::OnMainThread() ); // only call this from the frontend
+
 	if ( !tr.registered )
 	{
 		return;