8u40 (already reviewed) post-push notification: RT-36566 - make vram target limit more dynamic

Kevin already reviewed this fix during a discussion on the Jira issue (see the comments), but I realized after I pushed that I never sent out a public request for reviews to the list... Jira: https://javafx-jira.kenai.com/browse/RT-36566 webrev:

New vram usage algorithms, [was Re: 8u40 (already reviewed) post-push notification: RT-36566 - make vram target limit more dynamic]

Some interesting changes for developers to note with this fix: - Previously (pre-8.0) we added 2 parameters to control our vram usage: -Dprism.maxvram=NNN[kmg] -Dprism.targetvram=NNN[kmg] - Our implementation was fairly ham-handed, if you ever needed more than the target we would