Test using gl_ViewportIndex in a vertex shader

Test using gl_ViewportIndex in a vertex shader

Needs VK_EXT_shader_viewport_index_layer extension. Test works by
setting up N viewports forming a grid and each pair of triangles is
assigned to one of the viewports. Colors are used to distinguish each
pair, and by consequence, each cell in the grid.

This test has some similarities with how the multi_viewport scissors
test in vulkan/fragment_ops/.

New tests:
dEQP-VK.draw.shader_viewport_index.vertex_shader_*

VK-GL-CTS issue: 1043

Components: Vulkan

Change-Id: I311012126f0f4acc18cc31350e16f1c36dd35c62