Issue exposed by a transform feedback test.  Since we already create a
scissor to match the viewport (be it 0-sized or not), we can create the
Vulkan viewport with a small width/height if they are 0, as that's not
allowed by Vulkan.

Bug: angleproject:3205
Change-Id: Ib23d8b9f5e84447be2c4b7e8aff805e7d9e57323
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1665352
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>