pdf/ps: avoid outputting excess decimal places in matrices

Sometimes as a result of rounding errors in matrix transformations the
matrices in ps/pdf output look like:

    0.000000000000000061 1 1 -0.000000000000000061 0 842 cm

This patch rounds to zero matrix elements that are very small compared to
other elements in the same matrix.
Check for XRenderSolidFill()
Merge branch 'master' of git://anongit.freedesktop.org/git/cairo