Improve existing cull distance tests

Improve existing cull distance tests

Previously the tests were just writing a constant value to
gl_CullDistance[i] in the vertex shader and copying that value in
the rest of the geometry stages.

This commit does the following:

- The last shader writting to gl_CullDistance assigns 0 if
gl_Position.y < 0 and 0.5f otherwise. That way we can test the
interpolation of gl_CullDistance in the fragment shader.

- In case there are multiple geometry shaders, all except the last one
write a different value to gl_CullDistance[i] and checks the value
written by the previous shader stage. Therefore we check the values
are properly propagated between stages and they are overwritten as
well.

Components: Vulkan
VK-GL-CTS issue: 2961

Affected tests:

    dEQP-VK.clipping.*.clip_cull_distance*

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Change-Id: I213cc0469e72d82a8eda814e770aac2b825286fc