[Mesa-stable] Picking driconf improvements for 9.2

[Marek Olšák]

Hi Carl,

Could you please pick these commits over to the 9.2 branch if you're
okay with it? They enable the app-specific workarounds for all drivers
and add a few more. In this order:

2acc27cc6de5cae395d19017daf86ddd8de704cf
(st/dri: support the driconf option disable_blend_func_extended)

f27f3a4b15449e9ba3c0ee4e01b9db753e48e55f
(driconf: remove the unused option allow_large_textures)

83dbe61ea4308638f1c041d2f550f0f719e36967
(st/dri: implement the driconf option force_s3tc_enable properly)

772070527f6a6db72505575d6571470280a131ab
(st/dri: remove more unused driconf options)

7f2f804c75f6c7451c3630cb48e4675686bd68a7
(driconf: enable app-specific workarounds for all drivers)

0f6a7cb00c86fbdb415b01450bb1ece8cfe1e31d
(mesa,glsl,st/dri: add a new driconf option force_glsl_version for Unigine)

7568a89500c35f14cbd397f87c77acc915afc672
(st/dri: add a new driconf option disable_shader_bit_encoding for Unigine)

Thank you.

Marek