Change to use the middle button of the mouse in GUI mode #225

Job	Run time
Build and test g++-9 -	5m 24s
Build and test g++-10 -	5m 5s
Build and test clang++-10 -	4m 50s
Build and test clang++-11 -	5m 15s
Build and test g++-9 - -DPBRT_DBG_LOGGING=True	3m 56s
Build and test g++-10 - -DPBRT_DBG_LOGGING=True	3m 46s
Build and test clang++-10 - -DPBRT_DBG_LOGGING=True	3m 50s
Build and test clang++-11 - -DPBRT_DBG_LOGGING=True	3m 45s
Build and test g++-9 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 59s
Build and test g++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 55s
Build and test clang++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 52s
Build and test clang++-11 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 57s
	51m 34s

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