Implement query object interface.

This replaces the temporary occlusion counter functions we had before.
Added new ctx->Driver.WaitQuery() function which should block until the result is ready.
Sketch out some code for vertex transformation feedback counters.
diff --git a/src/mesa/drivers/common/driverfuncs.c b/src/mesa/drivers/common/driverfuncs.c
index 500dbb2..96e5037 100644
--- a/src/mesa/drivers/common/driverfuncs.c
+++ b/src/mesa/drivers/common/driverfuncs.c
@@ -224,8 +224,9 @@
 
    /* query objects */
    driver->NewQueryObject = _mesa_new_query_object;
-   driver->BeginQuery = NULL;
-   driver->EndQuery = NULL;
+   driver->BeginQuery = _mesa_begin_query;
+   driver->EndQuery = _mesa_end_query;
+   driver->WaitQuery = _mesa_wait_query;
 
    /* APPLE_vertex_array_object */
    driver->NewArrayObject = _mesa_new_array_object;