Fix text placement line hit detection for pixelRatio !== 1

[ahocevar]

This pull request makes the recently added tests for hit detection of line placement texts better readable.

This pull request fixes the pixel ratio problem identified by the new text fill tests.

[github-actions]

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[mike-000]

First test has no fill but detects wide stroke
Second test has no fill and thin stroke, detects nothing
Third test detects transparent fill

For the tests to pass use pixelRatio: 1 in the map options.

[ahocevar]

@mike-000 This looks fishy. Only the third test fails. If there is something wrong with pixel ratio handling for text hit detection, I would expect all three tests to fail on a device with a pixel ratio of 2.

[mike-000]

@mike-000 This looks fishy. Only the third test fails. If there is something wrong with pixel ratio handling for text hit detection, I would expect all three tests to fail on a device with a pixel ratio of 2.

The wide stroke in the first test would extend outwards as well as into the center of the character - just enough to get a hit a [50, 50] even though the hit detection image was misplaced. Increase the pixel ratio beyond 2 the misplacement increases and it also would fail.

[ahocevar]

@mike-000 Nevermind. I'm looking into a fix for the pixel ratio issue now.

[ahocevar]

This is now ready for review. For those who have been following: I re-purposed this pull request to not only make the tests more readable, but fix the pixel ratio issue I encountered when running the tests on a system with a device pixel ratio other than 1.

[tschaub]

Thanks for your work on this, @ahocevar