Allow rounding error when reading back Z buffer

Allow rounding error when reading back Z buffer

The early fragment test produces Z values <= 0.5 and uses a shader
to write 0.75 that it expects to be ignored due to early fragment
tests, which it verifies by reading back the Z buffer and testing
that all values are <= 0.5.

On our platform, we observe that on storing 0.5 we retrieve
0.500008, which makes us fail the test incorrectly. Fix this
by adding a small room for rounding error, like we do for
other verification paths in these tests.

While we are doing this, lower the tolerance admited for
the late fragment test case as well.

Components: Vulkan
VK-GL-CTS Issue: 3340
Affects:
dEQP-VK.fragment_operations.early_fragment.discard_early_fragment_tests_depth
dEQP-VK.fragment_operations.early_fragment.discard_no_early_fragment_tests_depth

Change-Id: I8e1782b81d0e74a4a5af3e3cac40c9679f543ac6