Fix helper_invocation.* rendering mostly black.

- Fix rendering viewport to match fbo dimensions to avoid rendering outside the
  viewport.
- Try to not generate invisible (fully clipped) triangles.
- Do not try to output negative dFdx and dFdy values to framebuffer.
- Detect and reject empty results.

Bug: 22066458
Change-Id: If3ec424ce10ee52c47b6fd28af125dc9e71c3a22
1 file changed