Fix signed integer overflow in angle_end2end_tests. The test math was causing an signed overflow in the most extreme case. Test: GLSLTest_ES3.GLVertexIDIntegerTextureDrawArrays/ES3_Vulkan_SwiftShader Bug: angleproject:5795 Change-Id: Id6b845577cb6ce585ec67251dcb6111d5773012f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3289942 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>