clutter/text: Check if attributes are equal before applying

Pango doesn't make it easy but the lists are generally extremely short and it's worth the effort. Because when they are equal we can avoid the clutter_actor_queue_relayout in clutter_text_set_attributes. This particularly avoids stuttering when moving the mouse over the gnome-shell calendar (which repeatedly sets font-feature-settings: "tnum" on calendar-day-base themed widgets).

Closes: gnome-shell#1411 (closed)

Before:

*** Performance over 1.0s: 51.07 FPS, update time avg: 4.1ms, peak: 16.1ms
*** Performance over 1.0s: 47.59 FPS, update time avg: 8.6ms, peak: 15.9ms
*** Performance over 1.0s: 43.79 FPS, update time avg: 8.5ms, peak: 16.0ms
*** Performance over 1.0s: 48.94 FPS, update time avg: 6.6ms, peak: 15.8ms

After:

*** Performance over 1.0s: 59.92 FPS, update time avg: 1.1ms, peak: 3.5ms
*** Performance over 1.0s: 59.91 FPS, update time avg: 2.1ms, peak: 3.3ms
*** Performance over 1.0s: 59.94 FPS, update time avg: 2.1ms, peak: 3.3ms
*** Performance over 1.0s: 59.97 FPS, update time avg: 2.3ms, peak: 3.3ms