RFR: 8238954: Improve performance of tiled snapshot rendering [v15]

[Frederic Thevenet]

On Wed, 1 Jul 2020 13:42:12 GMT, Ambarish Rapte <arapte at openjdk.org> wrote:

>> Frederic Thevenet has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   - Removed unused imports in Snapshot2Test
>>   - Fixed comments in QuantumToolkit
>>   - Only initialize RTT cache if needed
>
> Marked as reviewed by arapte (Reviewer).

I have read Ambarish's code more closely and I am now convinced it is indeed easier to understand; beyond the change
from "while" to "for" loops, it introduces an extra set of variables which avoids the same "m/b/rTileWidth/Height" vars
to have to designate different things depending in which loop they're used (M, B, or R). In hindsight, this is why I
had such a hard time naming them.

I have run the automated tests on Windows and Ubuntu (w/ forceGPU=true) and it passes. The benchmark I made also
produces similar results. Someone will still need to run the tests on macOS, I'm afraid.

If it's not too much of a problem now that the PR's been approved, I think it would beneficial to make the change,
especially considering that this bit of code might be used as a starting point for other issues requiring tiling.

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