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

[Kevin Rushforth]

On Mon, 29 Jun 2020 21:50:50 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> Frederic Thevenet has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fill test image with a bilinear gradient instead of random noise.
>
> 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.

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