mesa/es3: Add support for GL_PRIMITIVE_RESTART_FIXED_INDEX

mesa/es3: Add support for GL_PRIMITIVE_RESTART_FIXED_INDEX

This requires some derived state.  The cut vertex used is either the
value specified by glPrimitiveRestartIndex or it's hard-coded to ~0.
The derived state gl_array_attrib::_RestartIndex captures this value.
In addition, the derived state gl_array_attrib::_PrimitiveRestart is set
whenever either gl_array_attrib::PrimitiveRestart or
gl_array_attrib::PrimitiveRestartFixedIndex is set.

v2: Use _mesa_is_gles3.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>