Add Entity get_texture_mod() to Lua API

Send texture modifier to clients connecting later too

doc/lua_api.txt

@@ -2848,6 +2848,7 @@ This is basically a reference to a C++ `ServerActiveObject`
 * `set_yaw(radians)`
 * `get_yaw()`: returns number in radians
 * `set_texture_mod(mod)`
+* `get_texture_mod()` returns current texture modifier
 * `set_sprite(p={x=0,y=0}, num_frames=1, framelength=0.2,
   select_horiz_by_yawpitch=false)`
     * Select sprite from spritesheet with optional animation and DM-style