RFR: 8238954: Improve performance of tiled snapshot rendering [v13]

[Frederic Thevenet]

On Mon, 29 Jun 2020 22:46:26 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> I think I found the problem in the tiling logic that leads to the macOS failures. You need to check that the remainder
>> width or height is > 0. Also, it looks like you have the "B" and "R" loops backwards, which is a bit confusing.
>
> Actually, it also fails the same way on Ubuntu 20.04 as it does on macOS if I force HW acceleration on Ubuntu. It's off
> by default when running in a VM, so is using the SW pipeline, which doesn't have a hard limit (so that could explain
> the heap problem you were facing).  Ensuring that the tile size is > 0 fixes it on Linux and Mac.

I've pushed the changes discussed above, and verified that the tests now pass on a Ubuntu 20.04 VM with
`prism.forceGpu=true` (and that they indeed failed prior to those changes).

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PR: https://git.openjdk.java.net/jfx/pull/112