line-flat-clip-color: test flat-shaded line clipping color

This exercises a bug found in Mesa with softpipe, llvmpipe and swrast

Reviewed-by: José Fonseca <jfonseca@vmware.com>
Add test to verify interpolation at sample position

ARB_sample_shading extension says:
 "When the sample shading fraction is 1.0, a separate set of colors
  and other associated data are evaluated for each sample, each set
  of values are evaluated at the sample location."

Test passes on intel hardware with mesa patch: 'i965: Use sample
barycentric coordinates with per sample shading'.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
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:
centroid in vec4 b;
main()
{
  ...
}

Variable 'b' should be interpolated at sample's location.

V2: Use just one 'in' variable with the 'centroid' qualifier.
Another test already covers the interpolation of 'in' variable
without any 'qualifier'. This change also captures the mesa bug
reported on i965 driver:
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73915

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>