libretro: Dynamic Game Options

[RobLoach]

As part of conf(), provide a way to set custom game options.

#define RETRO_ENVIRONMENT_SET_VARIABLES 16
                                           /* const struct retro_variable * --
                                            * Allows an implementation to signal the environment
                                            * which variables it might want to check for later using
                                            * GET_VARIABLE.
                                            * This allows the frontend to present these variables to
                                            * a user dynamically.
                                            * This should be called the first time as early as
                                            * possible (ideally in retro_set_environment).
                                            * Afterward it may be called again for the core to communicate
                                            * updated options to the frontend, but the number of core
                                            * options must not change from the number in the initial call.
					    *
                                            * 'data' points to an array of retro_variable structs
                                            * terminated by a { NULL, NULL } element.
                                            * retro_variable::key should be namespaced to not collide
                                            * with other implementations' keys. E.g. A core called
                                            * 'foo' should use keys named as 'foo_option'.
                                            * retro_variable::value should contain a human readable
                                            * description of the key as well as a '|' delimited list
                                            * of expected values.
                                            *
                                            * The number of possible options should be very limited,
                                            * i.e. it should be feasible to cycle through options
                                            * without a keyboard.
                                            *
                                            * First entry should be treated as a default.
                                            *
                                            * Example entry:
                                            * { "foo_option", "Speed hack coprocessor X; false|true" }
                                            *
                                            * Text before first ';' is description. This ';' must be
                                            * followed by a space, and followed by a list of possible
                                            * values split up with '|'.
                                            *
                                            * Only strings are operated on. The possible values will
                                            * generally be displayed and stored as-is by the frontend.
                                            */