Fix KHR-Single-GL46.subgroups.* cases failure

Drawing a point at a coordinate with integer value will have different
behavior on variant vender's hardware.
So, as a fix, we could initialize the coordinate in the center of the
pixel.

Affects:
KHR-Single-GL46.subgroups.*

Components: OpenGL

VK-GL-CTS issue: https://gitlab.khronos.org/Tracker/vk-gl-cts/-/issues/4297

Change-Id: I1841306483ebe4b917670743e77f76fc393a3854

external/openglcts/modules/common/subgroups/glcSubgroupsTestsUtils.cpp

@@ -1326,7 +1326,7 @@ tcu::TestStatus glc::subgroups::makeGeometryFrameBufferTest(
 
 	{
 		glw::GLvoid *			bufferPtr			= vertexBuffer.mapBufferPtr();
-		std::vector<tcu::Vec4>	data				(maxWidth, tcu::Vec4(1.0f, 1.0f, 1.0f, 1.0f));
+		std::vector<tcu::Vec4>	data				(maxWidth, tcu::Vec4(1.0f, 0.5f, 1.0f, 1.0f));
 		const float				pixelSize			= 2.0f / static_cast<float>(maxWidth);
 		float					leftHandPosition	= -1.0f;
 
@@ -1780,7 +1780,7 @@ tcu::TestStatus glc::subgroups::makeVertexFrameBufferTest(Context& context, Form
 
 	{
 		glw::GLvoid *			bufferPtr			= vertexBuffer.mapBufferPtr();
-		std::vector<tcu::Vec4>	data				(maxWidth, tcu::Vec4(1.0f, 1.0f, 1.0f, 1.0f));
+		std::vector<tcu::Vec4>	data				(maxWidth, tcu::Vec4(1.0f, 0.5f, 1.0f, 1.0f));
 		const float				pixelSize			= 2.0f / static_cast<float>(maxWidth);
 		float					leftHandPosition	= -1.0f;