tu: Implement VK_EXT_post_depth_coverage

Passes:
 dEQP-VK.pipeline.monolithic.multisample.sample_mask_with_depth_test.*
 KHR-GL45.post_depth_coverage_tests.*

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <!19682>