[ftbench] New option `-a` to apply design coordinates.

The performance of MM fonts is quite different in comparison to static
fonts.  To be able to get benchmarks for the former, add a new command-line
option to pass design coordinates.

* src/ftbench.c: Include `ftmm.h`.
(MAX_MM_AXES): New macro.
(multimaster, design_pos, requested_pos, requested_cnt, used_num_axes): New
global variables.
(get_face): Load MM data if requested.
(parse_design_coords): New function.

* man/ftbench.1: Updated.