rendercheck: background should be cleared when drawing trapezoids.

[Vikram Fugro]

Hi all,

With the recent pixman library, trapezoid drawing on background has been fixed 
by clearing the background (instead of retaining the color of the background).

I have done the necessary changes in the triangles use case (in rendercheck) 
for the "color expected" from the background. 

Attaching the patch and the snapshots for reference.

Thanks,
Vikram
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