The finishes are calibrated against flutter_tester's stepped blur, not what a device delivers #39

Found while measuring #33 on devices. The finishes were calibrated headless, on flutter_tester's Skia raster, where the blur moves in steps: regular, asked for 2.6 texels, delivers 2.45 there, and 2.45 is what matched Apple's texture. On a device's GPU the blur does not step:

So on devices, Skia and Impeller alike, regular blurs about 2.6 texels, 5-7% more than the 2.45 that matched Apple's.

To decide: recalibrate the finishes' sigmas against what a device delivers (the meant sigma is the one that matched Apple, 2.45, not the one asked), and take the steps out of the tests' reference; or record it as within tolerance. Either way it needs the comparison against Apple's texture redone on a device.