FrameCapture: Warn when shaders are not attached yet Instead of assert, print a warning when we don't have attached shaders on an unlinked program. There's work involved to support it, but also scenarios where it isn't a blocker to capturing a trace. For instance, if the program isn't used, we don't care if it doesn't have attached shaders. Test: SimCity BuildIt Bug: b/334030316 Change-Id: Ied85af433e769ec8f1e436de82e911ff691ea9c0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6017396 Reviewed-by: Mark Łobodziński <mark@lunarg.com> Commit-Queue: Cody Northrop <cnorthrop@google.com>