Fix volume clip bug and add regression test.

When clipping a volume using in-shader clipping planes, it was possible for the starting point of the ray cast to lie beyond the data volume. The raycast code is written such that the first sample is always taken before testing termination criteria, and in these cases we would always take a single sample outside of the volume, leading to artifacts.

Fixed this behavior by checking that the starting position calculated by AdjustSampleRangeForClipping is indeed inside of the volume bounds and aborting the raycast if it is not.

The existing TestGPURayCastClipping test would have caught this, except that the vase.vti volume used for testing has all 0's at the boundaries, so the rendering was correct even with the edge-clamp repetition outside of the volume (the faulty samples always computed RGBA=vec4(0)). I replaced the vase.vti of this test with a wavelet with finite boundary values that will catch this problem if there's a regression.

Before:

After: