Merge pull request #306 from Nazg-Gul/dev

Fix compilation error with CLEW enabled
Code cleanup for GLFrameBuffer in examples/common

- cleanup some duplicated code
- change the background color ramp function to make UI more readable
Fixing broken build (forgot to remove WIP hbao from CMakeList)
Fix OSX Core Profile build

- remove obsolete glPushClient functions

note: this code is still duplicated in the ptexViewer (which still needs to be upgraded to the new framebuffer)

fixes #307
Fix doxygen generation errors
Update release notes for release 2.5.0
Release 2.5.0
Merge branch 'release/v2_5_0' into dev