gears: fix invalid calculation of the first FPS

gears: fix invalid calculation of the first FPS

At the calculation of the first FPS, gears has initialized last
FPS time with gettimeofday().
But the callback_data passed in the callback of wl_surface_frame()
is the current time, in milliseconds, with an undefined base.
Because of this subtracting last FPS time from callback_data makes no sense.
 For example, below is the result of running weston-gears on weston with
drm backend:

$ weston-gears
Warning: FPS count is limited by the wayland compositor or monitor refresh rate
1 frames in 1094460.125 seconds =  0.000 FPS
301 frames in  5.016 seconds = 60.008 FPS
301 frames in  5.016 seconds = 60.008 FPS
301 frames in  5.016 seconds = 60.008 FPS

As you can see, the the first FPS value is something odd.

This patch fixes it by initializing last FPS time with the callback_data passed in
the first callback.

Reviewed-by: Nils Chr. Brause <nilschrbrause@gmail.com>