Update createRGBInitData() to use/return std::vector instead of raw ptr
w/ "new." This prevents potential for memory leaks.

Bug: angleproject:2958
Change-Id: I4b1bb4ee293e171f56d1221a562e607ff8eb0fb3
Reviewed-on: https://chromium-review.googlesource.com/c/1375229Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>