Automatic update of common submodule

From 1a07da9 to fe1672e
bufferpool: refactor free_buffer

Make a do_free_buffer method to also decrements the number of allocated
buffers. Stop will now be successful when all buffers are freed.
buffer: remove wrong comment

Refcount and writability are not related for memory objects.
buffer: add a new flag to track memory changes

Add a flag to check if the memory changed in a buffer.
bufferpool: Use TAG_MEMORY to check memory before releasing

Tag allocated buffers with TAG_MEMORY. When they are released later,
only add them back to the pool if the tag is still there and the memory
has not been changed, otherwise throw the buffer away.
Add unit test to check various scenarios.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=724481
buffer: add function to check writability of memory

Check if memory is writable in a buffer and thus is exclusively owned by
this buffer.
bufferpool: only release buffers with writable memory

Check if the memory is writable before releasing the buffer into the
pool again.
Add unit test for this scenario.
docs: add some more new API do docs
docs: convert the examples to use gtk-doc markup, instead of docbook

The gtk-doc markup is less intrusive and better handled when creating docs for
language bindings. The titles (where used) where not adding much.
buffer: add return values to g_return_if_fail

FIxes previous commit.
docs: fix problems introduced by c068b225fef5a9bf0

- Fix failing build
- Drop added trailing whitespace
Automatic update of common submodule

From fe1672e to bcb1518
docs: use the new markdown for ordered list.

This was plain text that had all list items one after the other (including a
repeated number). Now it will atleast look good when processed with gtk-doc
1.20.
docs: Fix typos and remove unknown annotations

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=725517
.gitignore: Ignore gcov intermediate files

https://bugzilla.gnome.org/show_bug.cgi?id=725478