Add support for mapping analogs to D-pad again.

Use a cleaner approach which can feasibly work on all platforms.
Uses input_push/pop_analod_dpad to modify the joyaxis field before
polling input. This way, the RetroPad D-pad binds can inherit the
joyaxis fields from ANALOG_LEFT_X_PLUS and friends.

Also add support for the field in RGUI and saves settings.
Push analog dpad mode for autoconfig as well.
Check GL_RENDERER for ATI as well.
Expand config key map table; have overlay’s use it to look up key names.
Rename input_find_sk_key.

Matches up better with the conventions in input_common.
The sk-prefix was from SSNES days (how things linger on ... D:)
Safety checks for accessing video_poke function pointers
Merge pull request #481 from libretro/dpad-map

Analog D-pad mapping