[cairo] New per-scaled_font mutex to fix locking bugs

[Adrian Johnson]

Carl Worth wrote:
> So all that's in now. I even cleaned up the patch into 4 separate
> commits and I've pushed it out now.
> 
> Have fun, (especially with the new, more thread-friendly cairo),

I get the following errors when compiling the new, more thread-friendly
cairo:

cairo-font.c: In function '_cairo_toy_font_face_hash_table_lock':
cairo-font.c:238: error: 'CAIRO_LOCK_FILE' undeclared (first use in this
function)
cairo-font.c:238: error: (Each undeclared identifier is reported only once
cairo-font.c:238: error: for each function it appears in.)
cairo-font.c:238: error: 'CAIRO_LOCK_LINE' undeclared (first use in this
function)
cairo-font.c: In function '_cairo_font_reset_static_data':
cairo-font.c:474: error: 'CAIRO_LOCK_FILE' undeclared (first use in this
function)
cairo-font.c:474: error: 'CAIRO_LOCK_LINE' undeclared (first use in this
function)