Implement splineProperty

[davepagurek]

Resolves #6766

Changes:

• Replaces splineEnds(mode) with splineProperty('ends', mode)
• Replaces curveTightness(t) with splineProperty('tightness', t)
• Fixes a bug where curve tightness was not being correctly passed to shapes
• Adjusts the shift threshold in visual tests: I had bumped it up before to be lenient enough for text rendering in different environments, but this was making tests pass for a curve tightness test when they shouldn't. I've updated it to be lower by default, and able to be updated per-suite, so text tests can be more lenient.