glx/dri2: Add support for GLX_ARB_create_context_robustness

glx/dri2: Add support for GLX_ARB_create_context_robustness

Add the infrastructure required for this extension.  There is no
xserver support and no driver support yet.  Drivers can enable this be
advertising DRI2 version 4 and accepting the
__DRI_CTX_FLAG_ROBUST_BUFFER_ACCESS flag and the
__DRI_CTX_ATTRIB_RESET_STRATEGY attribute in create context.

Some additional Mesa infrastructure is needed before drivers can do
this.  The GL_ARB_robustness spec, which all Mesa drivers already
advertise, requires:

    "If the behavior is LOSE_CONTEXT_ON_RESET_ARB, a graphics reset
    will result in the loss of all context state, requiring the
    recreation of all associated objects."

It is necessary to land this infrastructure now so that the related
infrastructure can land in the xserver.  The xserver has very long
release schedules, and the remaining Mesa parts should land long, long
before the next xserver merge window opens.

v2: Expose robustness as a DRI2 extension rather than bumping
__DRI_DRI2_VERSION.

v3: Add a comment explaining why dri2->base.version >= 3 is also
required for GLX_ARB_create_context_robustness.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>