Cache TERRAIN on texture_t

- K_GetTerrainForTextureNum no longer performs a string
  lookup

src/k_terrain.c

@@ -2066,4 +2066,6 @@ void K_InitTerrain(UINT16 wadNum)
 			free(name);
 		}
 	}
+
+	R_ClearTextureNumCache(false);
 }

src/r_textures.c

@@ -27,6 +27,7 @@
 #include "p_setup.h" // levelflats
 #include "byteptr.h"
 #include "dehacked.h"
+#include "k_terrain.h"
 
 #ifdef HWRENDER
 #include "hardware/hw_glob.h" // HWR_LoadMapTextures