If Catalyst 9.12 for Linux
and its changes didn't deliver on what you wanted for your Christmas wishes, perhaps this change to their open-source driver does deliver on one of your hopes. The ATI R600/700 open-source 3D stack should now be OpenGL 2.0 compatible (compared to OpenGL 1.5 previously) and the GLSL (GL Shading Language) support should be "mostly" completed.
AMD's Richard Li recently introduced a commit
that enables GL2 and there has been other AMD/ATI 3D work going on too. The Radeon Feature Matrix
has also been updated to reflect the GLSL support now being mostly done for the ATI Radeon HD 2000/3000/4000 series and its OpenGL 2.0 compatibility.
Though even with OpenGL 2.0 support many games and other 3D intensive applications still won't be able to run well with the Mesa stack since these drivers still aren't heavily optimized for performance and there is other work to be addressed. Don't look to be running Unigine Heaven on Linux
with the ATI Radeon Mesa driver or any other Mesa3D drivers anytime soon.