Added tests for rasterizer discard
New tests were added to verify correct functionality when setting
rasterizerDiscardEnable to VK_TRUE in rasterizationStateCreateInfo.
Tests verify that primitives are discarded before rasterization by
comparing the rendered image to a blank reference image and ensuring
that no fragment shader invocations have occured.
New tests:
dEQP-VK.rasterization.discard.*
Components: Vulkan
VK-GL-CTS issue: 1149
VK-GL-CTS public issue: 100
Change-Id: Ided170a2831efaac7e393ce8cafb93269180b3d0