RTech: add constant for GPU driven texure streaming database file

This is a dummy file which enables the GPU driven texture streaming system, which loads/drops textures based on GPU feedback rather than a static texture streaming database file.

src/public/rtech/istreamdb.h

@@ -8,6 +8,8 @@
 #include "ipakfile.h"
 
 #define STBSP_FILE_EXTENSION "stbsp"
+#define STBSP_GPU_DRIVEN_FILE "gpu_driven"
+
 #define STBSP_NOMINAL_TEX_RES 4096
 
 struct MaterialGlue_t;