Antialiasing on webgl2 panics

[kpreid]

Enabling the antialiasing graphics option, on web, will panic. Filed wgpu issue about the panic: gfx-rs/wgpu#5263. However, If I understand correctly,

• the problem is using a multisampled texture as an input to a shader, and
• the fix for that issue will be disabling the ability to do so (without a DownlevelFlags that the wgpu webgl backend won't support), not making it work.

Therefore, we need to make antialiasing and bloom (or whatever features are actually conflicting) mutually exclusive on web.