Alpha to Coverage broken on Firefox due to disabled Antialiasing

[animech-sijo]

In v1.66.0, rendering on Firefox broke #5856, which got a workaround in v1.67.1. Since then, Alpha to Coverage has been broken in Firefox. Even in engine v2 today it's broken.

Instead of a 4-bit alpha, Alpha to Coverage now acts like Alpha Test:

1.65.5	1.67.1	2.9.2

This is an issue in the editor as well, as the viewport is always running the latest engine. Comparing Firefox with Edge:

Firefox	Edge

Example project: https://playcanvas.com/project/1387661

This needs to be fixed in both v1 and v2. While you're at it, please see if you can replace the previous Firefox workaround with a proper solution, since the two issues seem closely related.

[mvaligursky]

Yep, the reason is that we disable antialiasing as firefox is failing to run our implementation - details on the linked issue.