scratchpad: fix moving scratchpad window

From the source:

    When starting i3 initially (and after each change to the connected
    outputs), this function fixes the resolution of the __i3
    pseudo-output. When that resolution is not set to a function which
    shares a common divisor with every active output’s resolution,
    floating point calculation errors will lead to the scratchpad window
    moving when shown repeatedly.

fixes #632

src/main.c

@@ -657,6 +657,8 @@ int main(int argc, char *argv[]) {
         randr_init(&randr_base);
     }
 
+    scratchpad_fix_resolution();
+
     xcb_query_pointer_reply_t *pointerreply;
     Output *output = NULL;
     if (!(pointerreply = xcb_query_pointer_reply(conn, pointercookie, NULL))) {