PbufferSurfaceGLX: avoid creating (0, 0)-sized pbuffers It crashes on Mesa. BUG=angleproject:1188 Change-Id: I1e1e3e696bfda0c127b683a620cbaf302087e43b Reviewed-on: https://chromium-review.googlesource.com/309630 Tested-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>