Merge branch '284-image-early-fragment-tests' into 'vulkan-cts-1.0-dev'

Merge branch '284-image-early-fragment-tests' into 'vulkan-cts-1.0-dev'

Early fragment tests

Ported from OpenGL ES 3.1

Some changes wrt the original implementation:
- original test was categorized as "image", but I moved it to "pipeline"
  as it seemed more appropriate. (As a result branch name is slightly off now)
- uses SSBO/atomicAdd instead of image/imageAtomicAdd. Image use was an
  implementation detail and buffer is more straightforward to use.
- GLES had non-VBO cases which don't translate to Vulkan so were left out.

Fixes #284

See merge request !435