More specifically, crash occurs when viewport.Y is negative, and
scissoring isn't explicitly set by user. Problem was a combination
of computing invalid scissor bounds, and after clamping to these
bounds, proceeding to index buffers when yMin >= yMax. Fixed
both computation of scissor bounds, and making sure to early out
when yMin >= yMax to avoid OOB buffer indexing.

Bug: chromium:904276
Change-Id: I14580d7c4f0d9888c19e037b47113624a247ede1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29971Tested-by: Antonio Maiorano <amaiorano@google.com>
Presubmit-Ready: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>