MultiDraw Refactor Refactor MultiDraw* from general Context to different ContextImpl. Move general multiDraw code to renderer_utils.cpp. Bug: angleproject:3402 Change-Id: I85cb4b781afa2b3a8beb382a9c735910057f2ebe Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2268580 Commit-Queue: Shrek Shao <shrekshao@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>