Try to make sure that math insets have a properly set buffer_ member

Try to make sure that math insets have a properly set buffer_ member

Set the buffer of contents that is added to a MathData object through
MathData::insert() (both versions)
MathData::push_back()
asArray()

Also in math macros, initialize look_ with the relevant buffer.

This reduces the number of insets hat do not have a proper buffer.

See #13050 for discussion of this issue.

(cherry picked from commit f3c5ff9cb72c5231f1e1e81452e67d6f12dadecb)