Public bug reported:

Under certain circumstances, the Mate-Control-Center Appearance window simply 
segfaults when the font-tab is chosen.
Here is how to reproduce it (and work-around it).

1. Always open mate-appearance-properties from the CLI, rather than the
control-center - just to see the error message. When it does crash, it
always gives this line:

mate-appearance-properties: ../../../../src/cairo-scaled-font.c:459: 
_cairo_scaled_glyph_page_destroy: Assertion `!scaled_font->cache_frozen' failed.
Aborted (core dumped)

2. To get back into a good state, (i.e. so that opening the font control
allows us to use it, rather than crash), do the following:

- Open appearance-properties
- Select one of the "good" themes. Experimentally, these include BlueMenta, 
Quid,  Fog, AlaDelta
- Go to the fonts tab (it doesn't now crash)
- Close appearance-properties window.

3. To make it crash:

- Open appearance-properties
- Select one of the "bad" themes. Experimentally, these include Ambiance, 
Ambiant-Mate, Radiance, Radiant-Mate
- Go to the fonts tab (it doesn't (usually) crash this time)
- Close appearance-properties window.
- Open the appearance-properties window again
- Go to the fonts tab (it doesn't (usually) crash this time)
- It will segfault instantly.

I looked into specific cases that upset it, and the crash only happens
on certain choices of Fixed-width font:

- Terminus (my favourite for years) causes the crash  (various font sizes: 
6,9,17 all do)
- The others,  don't seem to: I tried:  Ubuntu Mono, Ubuntu Condensed, 
Monospaced, Courier, Courier 10 Pitch, Courier New, 

in other words, the following will cause a segfault: all conditions are
necessary

* One of the Ambience-derived themes
* The monospace-font being set to Terminus
* Opening the font-tab of mate-appearance-properties for the second time after 
setting the theme.

I have these packages, running under Ubuntu Mate, on Wily 64-bit:
libfreetype6:amd64         2.5.2-4ubuntu1 
mate-control-center      1.8.3+dfsg1-2 
xfonts-terminus               4.39-1  
libcairo2:amd64                1.14.2-2ubuntu1     (this is also the latest 
release from upstream)


This is perhaps the weirdest bug I've ever tracked down... so I hope I've given 
enough info to get this fixed.

** Affects: mate-control-center (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1464069

Title:
  mate-appearance-properties segfaults on font-tab in certain cases
  (Ambience + Terminus)

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