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
[Android] work around input bind modifications, fixes custom binds
This is really ugly and should be fixed
List all special keys in sample configuration file.

It's not at all clear that, for example, the key
for the digit 5 should be "num5" and not simply "5".
List all keys for clarity.
Merge pull request #484 from lkesteloot/master

List all special keys in sample configuration file.
(PS3) Try scaling analog values to -0x7FFF/0x7FFF range - still fiddly/broken
(Menu) Safety checks for accessing video_poke function pointers
(D3D9) Some cleanups
(XDK) Fixed default analog axes
(XDK D3D) Clean up XDK D3D context file
(XDK) Move xdk_d3d_generate_pp to xdk_ctx.c
(XDK D3D) Clean up unneeded extern for Xbox 1
(XDK Ctx) Set SwapEffect to 'discard' for both 360 and Xbox 1
(D3D XDK) Reimplement PresentationInterval for Xbox
(XDK) Make generate_pp more generic and more like make_d3dpp from
d3d9_pc.cpp