This change introduces the standard gamepad layout. This changes adds
these APIs:
* func HasGamepadStandardLayoutMapping
* func IsGamepadStandardButtonPressed
* func GamepadStandardAxisValue
* type StandardGamepadButton
* type StandardGamepadAxis
The standard gamepad layout is based on the web standard. See
https://www.w3.org/TR/gamepad/#remapping.
On desktops, the SDL's gamecontrllerdb.txt is used. If the gamepad is
listed in the text file, the mapping works. GLFW's mapping featrue is
not used.
On browsers, the property of a gamepad 'mapping' is used. When the
mapping value is 'standard', the gamepad is recognized to have the
standard mapping.
On mobiles, the implementation is still WIP.
Updates #1557
