methods the members are populated, the flat data size is calcualted, the data
buffer is allocated and the convenience pointers are set up automatically. With
initialize() an arbitrary configuration can be created, with
initialize_to_extended_joystick() a configuration is created that exactly
matches the data structure of the extended_joystick struct.
Having them here makes it easy to use the structure from a driver as well
without the need to set up everything manually.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@41888 a95241bf-73f2-0310-859d-f6bbb57e9c96
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