Validate missing fragment outputs for dual-source blending * A secondary fragment output must be declared when dual-source blending is enabled in WebGL contexts. * Omitting locations for multiple fragment outputs is not allowed in WebGL contexts. Bug: angleproject:1085 Change-Id: I57febdc02c9ccc571971a81b6671869f19b0aa96 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4834672 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>