!UPSTREAM: i965/gs: Generalize some error messages for GS.

TODO: find more places to generalize.
TODO: Don't use "VS/GS" terminology.  Use "vec4" terminology, since that will endure longer.
!UPSTREAM: glsl/linker: Fail if linking geometry shader with 0 GEOMETRY_VERTICES_OUT.

ARB_geometry_shader4 spec 2.15.2:
"Linking will also fail [...] if the program object contains objects to form a
geometry shader, and the value of GEOMETRY_VERTICES_OUT_ARB is zero."

Reviewed-by: Paul Berry <stereotype441@gmail.com>

TODO: May not be necessary for GLSL 1.50 style geometry shaders.  Make a test to validate.
!UPSTREAM: Work around bug in AntTweakBar

AntTweakBar tries to use glGetString(GL_EXTENSIONS) when connected to
a core context (it should use glGetStringi()).  Go ahead and allow
this for the time being.