A scaled font contains font options. Since commit 67eeed44, this can
contain an extra allocation for a custom palette. Since commit
edf9497c, this contains an extra allocation for a string. Before these
commit, font options could just be dropped, but now they need to be
freed.

This commit makes the relevant code for creating and finishing scaled
fonts also clean up the font options.

The test added in the previous commit also hits this bug (I only found
these leaks accidentially!). Running "valgrind --leak-check=full
./cairo-test-suite -f leaks-set-scaled-font" no longer reports the following
after this change:

 40 bytes in 1 blocks are definitely lost in loss record 1 of 11
    at 0x48407B4: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x4886C62: _cairo_font_options_init_copy (cairo-font-options.c:105)
    by 0x48DAFFB: _cairo_scaled_font_init_key (cairo-scaled-font.c:675)
    by 0x48DC077: cairo_scaled_font_create (cairo-scaled-font....