Add tolerance to conformance/canvas/to-data-url-test.html

WebGL and WebGL2 conformance tests conformance/canvas/to-data-url-test.html are failing on Chrome on mac with AMD GPUs, device IDs 0x6821, and 0x679e. Some of the tested pixels have alpha values that are off by one. Allowing a +/-1 tolerance to the test would be helpful to remove a test suppression.

See: https://crbug.com/1230781

The target is conformance/canvas/to-data-url-test.html; start by reading its pixel comparisons and the related Chrome suppression context. Confirm the WebGL and WebGL2 conformance cases for the reported AMD devices, and consider the issue done when the +/-1 alpha variance passes without suppression.

Written by the indexing model from the issue text.