css: Fix handling of transform values

css: Fix handling of transform values

When computing a transform value, there is nothing
to do, but we still need to copy the matrix from
src to dest, since it depends on the other transforms
in the array whether we are using the src or the
dest in the end.

This fixes cases like
-gtk-icon-transform: perspective(100px) matrix(1,2,...);
which would otherwise end up with a zero matrix.