invalidate UI render instance cache when freeing model instances

Fixes a startup assertion failure in Star Fox: Event Horizon, where a Lua
call to gr.freeAllModels() wiped Polygon_model_instances but left the UI
render instance cache holding stale indices. The next state transition
called model_clear_cached_ui_render_instances() which asserted in
model_delete_instance().

Closes #7513

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Goober5000

code/model/modelread.cpp

@@ -31,6 +31,7 @@
 #include "math/fvi.h"
 #include "math/vecmat.h"
 #include "model/model.h"
+#include "model/modelrender.h"
 #include "model/modelreplace.h"
 #include "model/modelsinc.h"
 #include "parse/parselo.h"
@@ -338,6 +339,10 @@ void model_instance_free_all()
 {
 	size_t i;
 
+	// invalidate the UI render instance cache first so it doesn't retain stale references
+	// into Polygon_model_instances after the loop below
+	model_clear_cached_ui_render_instances();
+
 	// free any outstanding model instances
 	for ( i = 0; i < Polygon_model_instances.size(); ++i ) {
 		if ( Polygon_model_instances[i] ) {