Add an optional parameter (ANGLE_CAPTURE_LABEL) to
ANGLE frame capture that tags files and functions
uniquely, allowing multiple frame captures to be
replayed by a test harness.

Example:

  ANGLE_CAPTURE_LABEL=foo

Results in filenames like this:

  foo_capture_context1.cpp
  foo_capture_context1.h
  foo_capture_context1_files.txt
  foo_capture_context1_frame000.angledata
  foo_capture_context1_frame000.cpp
  foo_capture_context1_frame001.angledata
  foo_capture_context1_frame001.cpp
  ...

Functions wrapped in namespaces like this:

  namespace foo
  {
      void ReplayContext1Frame0();
      void ReplayContext1Frame1();
  }

For use like this:

  foo::SetupContext1Replay();
  for (...)
  {
      foo::ReplayContext1Frame(i);
  }

Bug: angleproject:3630
Change-Id: Ibe27dc4d40a36606ee40678a9e4d43b5a4baf976
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1949603
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>