Antialiasing in SDF

[xiaoiver]

[prestonbourne]

I also noticed this issue.

it seems like it originates from the g-device-api library, https://stackoverflow.com/questions/11343820/webgl-semi-transparent-appears-white

Here is the culprit: https://github.com/antvis/g-device-api/blob/f5aea083fa23b0f293eab27264294e61c4c83a6f/src/webgl/WebGLDeviceContribution.ts#L26

I'll open a PR in this repo but i'm curious to learn what would be better. changing the default setting here? or changing the default in the HAL

EDIT:
Can't build locally due to some cryptic python dependency error? So I won't bother with the PR but I repro'd and changing the premultipliedAlpha seemed to fix it