Fix logic in GPUVolumeRayCastMapper GL support test

The logic was failing cards that had support, using
the extension manager un-initialized, and not reporting
the specifc reason for failure. This patch fixes these
issues.

Change-Id: If31d66e304cdea81e09a5db08726587fbc0853e0
Merge topic 'gpu-raycast-mapper-error-report' into master

4212d0af Fix logic in GPUVolumeRayCastMapper GL support test