Clarify linear scaling in ImageDataProvider contract

[arunjose696]

This PR extends ImageDataProvider contract to clarify that implementations need to yield linearly scaled image data.
This commit also extends the strict checks for verifying linear scaling to GTK and Cocoa implementations.

[github-actions]

Test Results

   546 files  ±0     546 suites  ±0   30m 40s ⏱️ - 5m 35s
 4 425 tests ±0   4 408 ✅ ±0   17 💤 ±0  0 ❌ ±0 
16 746 runs  ±0  16 619 ✅ ±0  127 💤 ±0  0 ❌ ±0 

Results for commit ac26789. ± Comparison against base commit 9620d00.

♻️ This comment has been updated with latest results.

[HeikoKlare]

The change looks good to me. I only have one comment regarding the system property usage on Linux/GTK.

[HeikoKlare]

The change looks fine now.
I just have one proposal: instead of adding this constant that can be used from everywhere, we might encapsulate it (make it private) and provide methods like runIfStrictChecksEnabled(Runnable) and runWithStrictChecksDisabled(Runnable) that represent the two use cases we currently have in consumers code. And maybe we could do that in a separate PR (preparatory for the contract improval), so that we have the two changes (improvement of strict checks and contract improvement) separated.

[arunjose696]

The change looks fine now. I just have one proposal: instead of adding this constant that can be used from everywhere, we might encapsulate it (make it private) and provide methods like runIfStrictChecksEnabled(Runnable) and runWithStrictChecksDisabled(Runnable) that represent the two use cases we currently have in consumers code. And maybe we could do that in a separate PR (preparatory for the contract improval), so that we have the two changes (improvement of strict checks and contract improvement) separated.

Have made the refactoring of strictchecks to another PR #2410. This PR now only has the contract extension

[HeikoKlare]

Sound good, thank you!

I just simplified the commit message as it still referred to the modification of strict checks which has been extracted to #2410