implemented support for Windows XInput (using different keys/cvars to allow non-confl...

implemented support for Windows XInput (using different keys/cvars to allow non-conflicting configs) in vid_shared.c, usable in WGL and SDL clients on Windows
redesigned joystick code to encode any joystick data into a vid_joystate_t struct and use a common code path for all handling of axes and button events (also rewrote joy_axiskeyevents as part of this)
changed joystick deadzone handling to make values start at 0 at the edge of the deadzone
removed legacy joystick code in vid_wgl.c because it was too annoying to maintain
removed some hacky SDL 1.3 iOS code that is no longer needed

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11154 d7cf8633-e32d-0410-b094-e92efae38249