Vulkan: Fix how the viewport is calculated with Y flip - This fixes all ViewportTest.* and MipmapTest.* - Tests left to fix in end2end: - BlitFramebuffer* - MaxTextureSizeTest.* - PointSpritesTest.* Every other test in angle_end2end_tests are working. Bug: angleproject:2673 Change-Id: I162083bc847c15fa5490ab524ad4c22747d232ea Reviewed-on: https://chromium-review.googlesource.com/1126333 Commit-Queue: Luc Ferron <lucferron@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>