GLES1State: Add getMatrixStack function. FrameCapture requires access to the whole matrix stack. This function also exposes stacks that are not currently set. Bug: angleproject:6181 Change-Id: I595feaa0cbd3e5a62e398aa7a0578ecf6425ffe6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3035049 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Lubosz Sarnecki <lubosz.sarnecki@collabora.com>