css: Track backdrop state separately

css: Track backdrop state separately

Like the previous change, this reduces the number of
css nodes reacting to parent-state changes.

Before (numbers from widget-factory with Adwaita):
2247 nodes
class 612
name 2246
id 2
first-child 148
last-child 156
state 2247
hover 562
disabled 859
sibling-name 63
sibling-disabled 51
parent-class 586
parent-name 788
parent-id 2
parent-first-child 78
parent-last-child 78
parent-state 236
parent-hover 5
parent-disabled 91

After:
2247 nodes
class 612
name 2246
id 2
first-child 148
last-child 156
state 2245
hover 562
disabled 859
backdrop 1080
sibling-name 63
sibling-disabled 51
parent-class 586
parent-name 788
parent-id 2
parent-first-child 78
parent-last-child 78
parent-state 236
parent-hover 5
parent-disabled 91
parent-backdrop 4