Don't truncate frame diffs in RtpGenericFrameDescriptor

RtpGenericFrameDescriptor::AddFrameDependencyDiff() took a uint16_t, so
frame diffs computed from 64-bit frame ids were silently truncated. A
diff that truncated into the valid range, e.g. 2^16 + 5, was accepted
and written as a dependency on the wrong frame.

Take the diff as int64_t and reject values that the descriptor can't
represent. RTPSenderVideo ignores the result, so it now leaves such a
dependency out rather than misrepresenting it. Leaving out the whole
descriptor instead would make the stream inconsistent, while the missing
dependency only matters to receivers that lost the referenced frame.
RtpDescriptorAuthentication calls the same method, so the authenticated
data still matches the descriptor that is sent.

Bug: webrtc:566342228
Change-Id: I344a641a1b0956984ed69fc44d439d8adba021df
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/505901
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48744}