NEWS: Summarize recent changes

This updates NEWS with items shown by `git log --stat 1.12.8..`
Hopefully I haven't misrepresented any of the work done.  For sake of
brevity where there were several changes in one area, I've given a
single general entry to summarize those changes.  The division between
features and bug fixes is quite rough.
pthread-same-source: Add ref image generation

When the new GENERATE_REFERENCE symbol is defined to 1, no threads are involved
at all. In this case, all operations are done sequentially by the main thread.

Thus, in this mode no thread issues can occur and the result must be usable as
reference images.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
pthread-same-source: Refresh reference images

These reference images are generated by the new GENERATE_REFERENCE mode that the
previous commit introduced.

I have no idea what the "base" images. From my reading of the code in
boilerplate/, these images will be used by the test-XXX targets. However, these
seem to generate the same result than e.g. the image backend. Thus, I deleted
these files.

There is still pthread-same-source.quartz.xfail.png. This file was created in
commit b6e16b8d and touched in commit 5a1e590b1. No idea if this is still valid
and since I don't have a Mac, I won't touch it.

The test is still broken on the following backends (out of the backends I have
compiled in). This mostly seems to be differences in image scaling, but I
couldn't figure out an easy way to tell the test suite that the new results are
correct.

test-paginated, ps2, ps3, xcb, xcb-window, xcb-window&, xcb-fallback, xlib,
xlib-window, xlib-fallback, recording

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