[css-transforms-1] Remove all uses of 4x4 matrices in transforms 1

[dirkschulze]

Level 1 still mentions 4x4 matrices a lot. Should be replaced and moved to transforms-2.

[smfr]

It's not clear that we want to define all the matrix operations in transforms-1 as 2D matrices, and then re-define them in 2 as 4x4. Tab thought that we should just use 4x4 matrices in transforms-1.

[smfr]

We agree to keep the use of 4x4 matrices in transforms-1, but to avoid any matrices with 3d components. Section 4.2 needs to be edited to avoid mentioning matrix3d(): https://drafts.csswg.org/css-transforms/#serialization-of-the-computed-value