Replaced use of lua_opt/checkint(eger) with lua_opt/checknumber

Using lua_check/opt/tonumber() assures a non-integer number is floored
when cast to an int, in contrast to lua_tointeger() which is "truncated
in some non-specified way."

src/mouse.c

@@ -99,7 +99,7 @@ int l_mouse_isDown(lua_State *L) {
   int res = 0;
   int i;
   for (i = 1; i <= n; i++) {
-    int idx = luaL_checkinteger(L, i) - 1;
+    int idx = luaL_checknumber(L, i) - 1;
     if (idx >= 0 && idx < MOUSE_BUTTON_MAX) {
       res |= mouse_buttonsDown[idx];
     }