Update verification threshold in the depth bias tests

Verification depth threshold in the depth bias tests is too strict to
pass on some devices. Updating the threshold while it is still below
the depth delta in each pixel resolves the issue.

Affects:
dEQP-VK.draw.*.inverted_depth_ranges.*

Components: Vulkan
VK-GL-CTS issue: 5079

Change-Id: I2d537e9004ab4a4670c35775f52ec1b15bc261c6

external/vulkancts/modules/vulkan/draw/vktDrawInvertedDepthRangesTests.cpp

@@ -75,7 +75,7 @@ constexpr float				kClearDepth		= 1.0f;
 constexpr int				kClearStencil	= 0;
 constexpr int				kMaskedStencil	= 1;
 constexpr float				kDepthEpsilon	= 0.00025f;	// Used to decide if a calculated depth passes the depth test.
-constexpr float				kDepthThreshold	= 0.0025f;	// Used when checking depth buffer values. Less than depth delta in each pixel (~= 1.4/205).
+constexpr float				kDepthThreshold = 0.0064f;	// Used when checking depth buffer values. Less than depth delta in each pixel (~= 1.4/205).
 constexpr float				kMargin			= 0.2f;		// Space between triangle and image border. See kVertices.
 constexpr float				kDiagonalMargin	= 0.00125f; // Makes sure the image diagonal falls inside the triangle. See kVertices.
 const Vec4					kVertexColor	(0.0f, 0.5f, 0.5f, 1.0f); // Note: the first component will vary.