Fix validation problems with renderpass tests

Fix validation problems with renderpass tests

This commit fixes several problems found while running the set of
renderpass tests with an up-to-date version of the validation layers.

* Stencil aspects of some images were not being properly transitioned to
  the expected layout used later in some tests.

* Fixed reported interface mismatch between vertex and geometry shaders.

* Invalid flush size for the vertex buffer in some tests, caused by not
  properly honoring nonCoherentAtomSize.

* In the same case as above, the allocation offset was being rounded up
  to the nearest multiple of nonCoherentAtomSize instead of being
  rounded down, which means some vertices could be left unflushed.

In addition, de::roundUp and de::roundDown are introduced by the commit
as they are somewhat frequent operations in some tests, but only updates
the affected renderpass tests to use them.

Affected tests:
dEQP-VK.renderpass.*

Components: Framework, Vulkan
VK-GL-CTS issue: 2307

Change-Id: Ic78b2fe4707c4ddb0ade91d85e124190e94a1ed2