font rendering crash

seems to be a sporadic failure. current sha1: 89e2ba08883ab381

gdb session:

```
andy@andy-bx:~/dev/genesis/build$ make && gdb ./genesis -ex run -ex quit
[ 13%] Built target libgenesis_shared
[ 14%] Built target delay
[ 18%] Built target generate_unicode_data
Scanning dependencies of target genesis
[ 19%] Building CXX object CMakeFiles/genesis.dir/src/genesis_editor.cpp.o
Linking CXX executable genesis
[ 60%] Built target genesis
[ 74%] Built target libgenesis_static
[ 75%] Built target list_devices
[ 77%] Built target list_supported_formats
[ 78%] Built target normalize_audio
[ 79%] Built target play
[ 79%] Built target rucksack_bundle
[ 80%] Built target synth
[100%] Built target unit_tests
GNU gdb (Ubuntu 7.9-1ubuntu1) 7.9
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Reading symbols from ./genesis...done.
Starting program: /home/andy/dev/genesis/build/genesis
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffde560700 (LWP 13578)]
[New Thread 0x7fffded61700 (LWP 13577)]
[New Thread 0x7fffe4f50700 (LWP 13576)]
[New Thread 0x7fffe9752700 (LWP 13575)]

Program received signal SIGSEGV, Segmentation fault.
0x0000000000427598 in HashMap::key_to_index (
this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:221
221 return (int)(HashFunction(key) % ((uint32_t)_capacity));
A debugging session is active.

Inferior 1 [process 13571] will be killed.

Quit anyway? (y or n) n
Not confirmed.
(gdb) up
#1 0x00000000004272d1 in HashMap::internal_get
(this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:206
206 int start_index = key_to_index(key);
(gdb)
#2 0x00000000004270bb in HashMap::maybe_get (
this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:70
70 return internal_get(key);
(gdb)
#3 0x0000000000426c92 in FontSize::font_cache_entry (this=0x1, codepoint=85)
at /home/andy/dev/genesis/src/font_size.cpp:38
38 auto *entry = _font_cache.maybe_get(codepoint);
(gdb)
#4 0x0000000000421e5e in Label::update (this=0x791118) at /home/andy/dev/genesis/src/label.cpp:112
112 FontCacheValue entry = _font_size->font_cache_entry(ch);
(gdb)
#5 0x000000000043406c in MenuWidgetItem::set_caption (this=0x791110, caption=..., new_mnemonic_index=0)
at /home/andy/dev/genesis/src/menu_widget.cpp:68
68 label.update();
(gdb)
#6 0x0000000000434010 in MenuWidgetItem::set_caption (this=0x791110, orig_caption=...)
at /home/andy/dev/genesis/src/menu_widget.cpp:63
63 return set_caption(caption, mnemonic_index);
(gdb)
#7 0x000000000042c851 in GenesisEditor::refresh_menu_state (this=0x75a8c0)
at /home/andy/dev/genesis/src/genesis_editor.cpp:251
251 editor_window->undo_menu->set_caption(undo_caption);
(gdb) print undo_caption
$1 = {static max_codepoint = 2097151, _chars = {_items = 0xd335a0, _length = 5, _capacity = 16}}
(gdb) print undo_caption.encode()
$2 = {_buffer = {_items = 0xa762f0 "&Undo", _length = 6, _capacity = 16}}
(gdb) down
#6 0x0000000000434010 in MenuWidgetItem::set_caption (this=0x791110, orig_caption=...)
at /home/andy/dev/genesis/src/menu_widget.cpp:63
63 return set_caption(caption, mnemonic_index);
(gdb)
#5 0x000000000043406c in MenuWidgetItem::set_caption (this=0x791110, caption=..., new_mnemonic_index=0)
at /home/andy/dev/genesis/src/menu_widget.cpp:68
68 label.update();
(gdb)
#4 0x0000000000421e5e in Label::update (this=0x791118) at /home/andy/dev/genesis/src/label.cpp:112
112 FontCacheValue entry = _font_size->font_cache_entry(ch);
(gdb)
#3 0x0000000000426c92 in FontSize::font_cache_entry (this=0x1, codepoint=85)
at /home/andy/dev/genesis/src/font_size.cpp:38
38 auto *entry = _font_cache.maybe_get(codepoint);
(gdb)
#2 0x00000000004270bb in HashMap::maybe_get (
this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:70
70 return internal_get(key);
(gdb)
#1 0x00000000004272d1 in HashMap::internal_get
(this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:206
206 int start_index = key_to_index(key);
(gdb)
#0 0x0000000000427598 in HashMap::key_to_index
(this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:221
221 return (int)(HashFunction(key) % ((uint32_t)_capacity));
(gdb)
Bottom (innermost) frame selected; you cannot go down.
(gdb) up
#1 0x00000000004272d1 in HashMap::internal_get
(this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:206
206 int start_index = key_to_index(key);
(gdb) print key
$3 = (const unsigned int &) @0x7fffffffda3c: 85
(gdb) up
#2 0x00000000004270bb in HashMap::maybe_get (
this=0x9, key=@0x7fffffffda3c: 85) at /home/andy/dev/genesis/src/hash_map.hpp:70
70 return internal_get(key);
(gdb) up
#3 0x0000000000426c92 in FontSize::font_cache_entry (this=0x1, codepoint=85)
at /home/andy/dev/genesis/src/font_size.cpp:38
38 auto *entry = _font_cache.maybe_get(codepoint);
(gdb) print codeponit
No symbol "codeponit" in current context.
(gdb) print codepoint
$4 = 85
(gdb) print _font_cache
Cannot access memory at address 0x9
(gdb) print *this
Cannot access memory at address 0x1
(gdb) up
#4 0x0000000000421e5e in Label::update (this=0x791118) at /home/andy/dev/genesis/src/label.cpp:112
112 FontCacheValue entry = _font_size->font_cache_entry(ch);
(gdb) print *this
$5 = {_gui = 0x100003503, _width = 0, _height = 0, _texture_id = 0, _vertex_buffer = 0, _text = {
static max_codepoint = 2097151, _chars = {_items = 0xd769a0, _length = 4, _capacity = 16}}, _font_size = 0x1,
_img_buffer = {_buffer = {_items = 0xaf7b50 "\240e{", _length = 2, _capacity = 16}}, _letters = {_items = 0x0,
_length = 0, _capacity = 0}}
(gdb) quit
A debugging session is active.

Inferior 1 [process 13571] will be killed.

Quit anyway? (y or n) y
```