ENH: Volume Mapper benchmark test

Change-Id: I2d91839bbd0f71325f722b4702f06b7f2dd017bf
FIX: Make sure context is present

Change-Id: Ic6bab39b44c9aa79f8181aee0b97bcbe4ad3a5b3
FIX: Bypass testing if OpenGL extensions not supported

Change-Id: I30b9e239b2c79fcd6077d13df05790ff68a7c408
Merge topic 'volume-rendering-benchmark' into master

3f787161 FIX: Bypass testing if OpenGL extensions not supported
b3abe909 FIX: Make sure context is present
933c76ab ENH: Volume Mapper benchmark test