Fix bug in diff generator

[DanielJette]

What does this change accomplish?

Resolves #215

I noticed a bug where the parallel processor was writing pixels to effectively random locations.

How have you achieved it?

Use put with index for parallel pixel processor transform function. The existing code only worked for sequential (single-core) operations. We must store the transformed pixel in the right location using the put method that takes a destination index. The other version just writes the values sequentially.

Tophat instructions

1. Run ScreenshotRuleExampleTests.generateDiffs
2. Pull screenshots from the device (./gradlew screenshotPull)
3. Verify that ScreenshotRuleExampleTests_generateDiffs.diff.png looks like what is expected

Notice

This change must keep master in a shippable state; it may be shipped without further notice.