[css-transforms] need two identity function concepts

[smfr]

Need two id() function concepts in the spec:

• one that matches any function
• one that only matches "like" functions (for use with perspective(0), rotate3d(x,x,x,0) etc.

[fantasai]

I think we want to refrain from calling it “id” since that is used widely in the Web platform to talk about element IDs, which are very different from identity functions.