Add test to verify 'centroid' qualifier is ignored in case of persample shading

Here is what it tests:
Enable sample shading for the whole fragment shader using OpenGL API
glEnable(GL_SAMPLE_SHADING);
glMinSampleShading(1.0);

fragment shader:
in vec4 a;
centroid in vec4 b;
main()
{
  ...
}

Both variables 'a' & 'b' should be interpolated at sample's location.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: Chris Forbes <chrisf@ijw.co.nz>
Rename Fbo to piglit_fbo and FboConfig to piglit_fbo_config

These name changes follow the naming convention used in other
shared piglit variables, functions and classes.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Rename shared test pattern classes and functions

Name changes in this patch follow the naming convention used in other
shared piglit variables, functions and classes.
Changes:
ManifestProgram to piglit_manifest_program
ManifestStencil to piglit_manifest_stencil
ManifestDepth to piglit_manifest_depth

TestPattern to piglit_test_pattern
Points to piglit_points
Lines to piglit_lines
Triangles to piglit_triangles
InterpolationTestPattern to piglit_interpolation_test_pattern

Sunburst to piglit_sunburst
StencilSunburst to piglit_stencil_sunburst
DepthSunburst to piglit_depth_sunburst
ColorGradientSunburst to piglit_color_gradient_sunburst

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>