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.