Clear all Views of Framebuffer Attachment to reference background color

Clear all Views of Framebuffer Attachment to reference background color

The VK_KHR_multiview spec defines that during implicit clear (LOAD_OP_CLEAR)
all Active Views of Framebuffer attachment are cleared.
ViewMask defines which Views are active during particular subpass,
therefore a View is considered active during RenderPass
if it was active in any subpass of that RenderPass.

Without explicit clear of inactive Views their content will be undefined,
making test results unreliable.

This change removes MultiViewReadbackTestInstance::beforeDraw() and
relies on MultiViewRenderTestInstance::beforeDraw()
to clear all Views (array layers) of framebuffer attachment
by calling vkCmdClearColorImage().

Components: Vulkan

VK-GL-CTS Issue: 1085

Affects:
    dEQP-VK.multiview.readback*

Change-Id: I70d8b1c48cb157df2518b7d241a302c54d5842f3