mesa: Restore depth texture state on glPopAttrib(GL_TEXTURE_BIT).

mesa: Restore depth texture state on glPopAttrib(GL_TEXTURE_BIT).

According to Table 6.17 in the GL 2.1 specification, DEPTH_TEXTURE_MODE,
TEXTURE_COMPARE_MODE, and TEXTURE_COMPARE_FUNC need to be restored on
glPopAttrib(GL_TEXTURE_BIT).

Makes a number of oglconform tests happier.

v2: Make restoration conditional on the ARB_shadow and ARB_depth_texture
    extensions, as suggested by Brian.  I'm not sure that any
    implementations still remain that don't support those, but why not?

NOTE: This is a candidate for stable release branches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>