Add enviroment callback to enable cores to notify the frontend that a core option value has changed

[jdgleaver]

Description

At present, changes to core option values are unidirectional: frontend -> core.

As requested by twinaphex, this PR adds a new RETRO_ENVIRONMENT_SET_VARIABLE callback which can be used by a core to transmit option value changes back to the frontend. Possible applications of this are:

• To ensure that frontend and core are properly synchronised when core option values can be set via an internal core menu (e.g. this would allow options currently set by PrBoom's in-game menu to be added to the frontend core options interface)
• To enable a core to force-set certain options when the user-set value of one option is incompatible with the current setting of another

The relevant API definition is as follows:

#define RETRO_ENVIRONMENT_SET_VARIABLE 70
                                           /* const struct retro_variable * --
                                            * Allows an implementation to notify the frontend
                                            * that a core option value has changed.
                                            *
                                            * retro_variable::key and retro_variable::value
                                            * must match strings that have been set previously
                                            * via one of the following:
                                            *
                                            * - RETRO_ENVIRONMENT_SET_VARIABLES
                                            * - RETRO_ENVIRONMENT_SET_CORE_OPTIONS
                                            * - RETRO_ENVIRONMENT_SET_CORE_OPTIONS_INTL
                                            * - RETRO_ENVIRONMENT_SET_CORE_OPTIONS_V2
                                            * - RETRO_ENVIRONMENT_SET_CORE_OPTIONS_V2_INTL
                                            *
                                            * After changing a core option value via this
                                            * callback, RETRO_ENVIRONMENT_GET_VARIABLE_UPDATE
                                            * will return true.
                                            *
                                            * If data is NULL, no changes will be registered
                                            * and the callback will return true; an
                                            * implementation may therefore pass NULL in order
                                            * to test whether the callback is supported.
                                            */

In addition, hash keys are now used to identify core option values to reduce overheads when performing lookups.

An example of the usage of this callback is provided here: libretro/gambatte-libretro#204

[i30817]

Can this solve the problem of the first run of mupen64plus attempting to use rdp plugin = gliden64 for the parallel renderer in a vulkan machine @m4xw ?