app: Use SHM transport for data transfer for display

Recent Cairo uses SHM transports when available, and exposes the ability
for its users to manage images shared between it and the display.
This allows us to eliminate copies, and if the architecture supports it
even to upload directly into GPU addressable memory without any copies
(all in normal system memory so we suffer no performance penalty when
applying the filters). The caveat is that we need to be aware of the
synchronize requirements, the cairo_surface_flush and
cairo_surface_mark_dirty, around access to the transport image. To
reduce the frequency of these barriers, we can subdivide the transport
image into small chunks as to satisfy individual updates and delay the
synchronisation barrier until we are forced to reuse earlier pixels.

Note this bumps the required Cairo version to 1.12, and please be aware
that the XSHM transport requires bug fixes from cairo.git (will be
1.12.12)

v2: After further reflections with Mitch, we realized we can share the
transport surface between all canvases by attaching it to the common
screen.

v3: Fix a couple of typos in insert_node() introduced when switching
variables names.

v4: Encapsulating within an image surface rather than a subsurface was
hiding the backing SHM segment from cairo, causing it to allocate
further SHM resources to stream the upload. We should be able to use a
sub-surface here, but it is more convenient to wrap the pixels in an
image surface for rendering the filters (and conveniently masking the
callee flushes from invalidating our parent transport surface).

Cc: Michael Natterer <mitch@gimp.org>