Bug #704925 "Illegal 'cm' inside text block gives incorrect result"

Because the old PDF interpreter applies the text matrix, text line
matrix, horizontal scaling and text rise at the time it encounters a 'BT'
operator, a subsequent 'cm' will cause the result to be incorrect if any
of these values are not the identity. This is because matrix algebra is
not commutative, applying the multiplication in the wrong order will
give the wrong result.

This commit addresses this (illegal) case by detecting that a 'cm' is
inside a text block, undoing the application of the text parameters
then applying the CTM change, before re-applying the text parameters.