fix coredumps in XRenderComputeTrapezoids (issue #1)

Rationale: I don't have enough expertise to judge on how the tessellation
algorithm is broken in XRenderComputeTrapezoids but I do trust Keith Packard
that it is. However using cairo for proper tessellation, as he suggests, is
too heavyweight, and here I propose to alter the code to at least do not cause
coredumps.

Even if/when the function will be marked as obsolete, I believe it is pretty
much capable of rendering relatively simple shapes, and still retains some
value.
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