Fix buffer alignment math in draw validation. We had a bug where we would require "stride * primCount" bytes when drawing from a buffer, when the last element would not require the full stride. The correct expression: "stride * (primCount-1) + size. BUG=angleproject:1086 TEST=dEQP-GLES3.functional.vertex_arrays.single_attribute.strides.* Change-Id: I7e2897c9d18b0e9849289d033f0ae2573237e4e5 Reviewed-on: https://chromium-review.googlesource.com/287320 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>