i965: Add GS support to INTEL_DEBUG=shader_time.

Previously, time spent in geometry shaders would be counted as part of
the vertex shader time.
i965: Modify some error messages to refer to "vec4" instead of "vs".

These messages are in code that is shared between the VS and GS
back-ends, so use the terminology "vec4" to avoid confusion.
!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.