Was used in the mip map ANGLE sample. Add a new helper function that we
can use for multiple captures.

Bug: angleproject:3611
Change-Id: I430ce0089df4ce81ba2ca2ad64c0c206b59397b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1894144
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>