Only reset world when button is actually pressed.

The VRUI button events are sent continuously, not only on state
change. The grab world button handler was resetting the modelview
matrix every time the corresponding button had an event, triggering
a reset for every frame of input data.

Change-Id: I205b8d9dac47ac44c2152571b4fcc3d16d70bf3c
Merge topic 'vr-fixInteractors' into next

fbd0080 Only reset world when button is actually pressed.