Use FixedVector for storing textures referenced by a framebuffer. This improves the performance of Framebuffer::hasTextureAttachment 3X. BUG=angleproject:2188 Change-Id: I6acc7f4440fe0232438182274d7bba6075530f38 Reviewed-on: https://chromium-review.googlesource.com/1159171 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>