Flush the texture staged updates when destroying context share group If we are using the extension EGL_ANGLE_display_texture_share_group, flush the texture staged updates upon destroying the context. With the extension enabled, the texture could still be alive when both context and its' EGL::ShareGroup are destroyed. If we have staged updates not yet flushed, the updates will keep the ShareGroupVk bufferpool occupied, causing an error upon ShareGroupVk::onDestroy(). Bug: chromium:1299211 Change-Id: I260de93c3a3099e023e31acbe017803e824459ad Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3495879 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Yuxin Hu <yuxinhu@google.com>