TextureGL: Signal DirtyBitsChanged on dirty bits. This is a refactoring change only in preparation for a change in the Vulkan back-end. We'll need to split the handling for Vulkan and GL into different functions. Bug: angleproject:4517 Change-Id: I2ddf9c70fe247d1dcb4e0aa9ca929bbbd61bd42b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2135893 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>