meta: Allow meta operations to pause/resume an active occlusion query

meta: Allow meta operations to pause/resume an active occlusion query

This allows for avoiding the occlusion query erroneously accumulating results
during the meta operation. This functionality is made conditional on a new
MESA_META_OCCLUSION_QUERY bit so that meta-operations which should generate
fragments can continue to get the current behavior.

The implementation of glClear is specifically augmented to request the flag
since glClear is specified to not generate fragments.

This fixes the following es3conform tests:

	occlusion_query_draw_occluded.test
 	occlusion_query_clear
	occlusion_query_custom_framebuffer
	occlusion_query_stencil_test
	occlusion_query_discarded_fragments

As well as the following piglit test:

	occlusion_query_meta_no_fragments

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