Breaking changes

• Renamed buttons RESERVED0 and RESERVED1 to TRIGGER and GRIP.

Behavioral changes

• GVR_CONTROLLER_ENABLE_POSITION is used to request real position data on
  systems that provide 6DoF controller data.

• gvr_controller_get_default_options() enables
  GVR_CONTROLLER_ENABLE_POSITION by default. This flag will disable
  GVR_CONTROLLER_ENABLE_ARM_MODEL on devices where the system provides 6DoF
  controller data to an app. If the app wants to ignore 6DoF controller data
  from the system and continue using GVR_CONTROLLER_ENABLE_ARM_MODEL, it
  should make sure GVR_CONTROLLER_ENABLE_POSITION is not set.
  GVR_CONTROLLER_ENABLE_POSITION has no effect on systems that only provide
  3DoF controller data.

Beta APIs for experimental 6DoF controllers

• Added Beta API support for
  experimental 6DoF Daydream controllers.

• Added new ndk-hellovrbeta sample. This sample is similar to ndk-hellovr but is focused on demonstrating Beta APIs such as the new 6DoF Daydream controller API.

NOTE: Beta APIs declared in the gvr_beta.h headers are not subject to the
usual deprecation process. Beta APIs are expected to change and may be removed
entirely in a future version without warning.