pdf: avoid making groups a transparency group if not required

If the group contains only a combination of clear and opaque alpha and
only OPERATOR_OVER is used in the group and to paint the group, a
transparency group is not required. This allows the pdf viewer to
replay the group in place.
recording: Fix unitialized variable 'free_me'

This variable should be NULL by default and gets set only if a new reference to
a surface was acquired through _cairo_surface_snapshot_get_target(). This works,
because cairo_surface_destroy(NULL) is well-defined and doesn't do anything.

Fixes the following compiler warning:

cairo-recording-surface.c:1642:24: warning: 'free_me' may be used uninitialized
in this function [-Wmaybe-uninitialized]

Signed-off-by: Uli Schlachter <psychon@znc.in>
recording: Correctly determine alpha of all surfaces

This code has special cases for recording and image surfaces. For all other
kinds of source surfaces, has_bilevel_alpha was not modified, even though the
source surface could have an alpha channel.

Fix this by using the same checks as in the general path at the end of this
function.

Signed-off-by: Uli Schlachter <psychon@znc.in>