Bug 1751053 - Avoid intermediate surface allocation to premultiply in...

Bug 1751053 - Avoid intermediate surface allocation to premultiply in ClientWebGLContext::GetSurfaceSnapshot. r=jgilbert, a=RyanVM

The intermediate allocation could fail and it was not handled, causing
an infrequent OOM crash. This patch makes it so we just reuse the
surface buffer we have and does the premultiply inline without requiring
an additional allocation.

Differential Revision: https://phabricator.services.mozilla.com/D136410