Intel GMA500 chipsets have a bad history when it comes to driver support.
The reason for this mess is basically that Intel introduced a new low-power SoC with an efficient CPU and a GPU by PowerVR. These GPUs were mainly used on mobile phones these days and were also very energy efficient.
However, there was a lot of complications about the question who takes the responsibility for the GPU driver – Intel or PowerVR?
Long story short: Intel was trying to blame PowerVR for the mess. But it seems that the industry made enough pressure, so Intel EMGD (Intel IMGD before) was introduced to deliver all binary blobs, all documentions for customisation and the DRM driver as source code.
Since the binary blobs depend on certain version-specific dependencies, I began to downgrade specific parts of my Linux installation, patched the DRM driver and made packages for easy installation. The only part that was not fully working was 2D acceleration via libVA on VLC or any other application than XBMC/Kodi.
(2019-04) Recently community members got active on continuing my work. I’m reviewing their patches and give them hints on fixing issues now. Personally I don’t have my ASUS T91 anymore because the battery was blowing up slowly and it was laying for years around.