Issue #894
sports a file with a diagonal tiling pattern matrices with
diagonal entries in the range of 5e-4.  While entries of this
size are unusual but okay, the determinant is below the rather
arbitrary threshold of 1e-6.  Therefore, poppler decided that
the matrix is singular and aborts the rendering.

Fix this by really only aborting if inverting the determinant
(which is the first thing that is being done with it) results
in a non-finite number.

As a side effect the code now also allows pattern matrices
with a negative determinant.  This does not seem to appear
in the wild all that often, but I didn't find anything
in the spec that rules it out.

BUG: #894