fonts: Fix regression(s) caused by c23ce62.

The builtin sysfont does not have an associated buflib_alloc_data
(because it's builtin right?). font_get_{width,bits} accessed a field of
it for all fonts which crashed on some systems but not on mine.
Solution: Move this field to struct font directly.

The cache size calculated was also busted.

Fixes FS#12944 and most likely FS#12938.

Change-Id: I32303c4335a12a6c421fdca34f7ece851aac12ca

firmware/export/font.h

@@ -107,6 +107,7 @@ struct font {
     unsigned char *buffer_position; /* position in the buffer */    
     unsigned char *buffer_end;      /* end of the buffer */
     size_t         buffer_size;     /* size of the buffer in bytes */
+    bool         disabled;        /* font disabled (use blank as fallback if not in cache) */
 #ifndef __PCTOOL__    
     struct font_cache cache;
     uint32_t file_width_offset;    /* offset to file width data    */