BUG: MNE_BROWSER_PRECOMPUTE=true broken

[larsoner]

On my system I have a lot more memory compared to size of files that I open, so for almost all files precomputation runs.

In theory this is a good thing, but in practice I've noticed some weird bugs where the display doesn't always update properly when I click and unclick bad channels, or enable/disable projectors.

I think relatedly, in #268 the updates only happened properly when I had MNE_BROWSER_PRECOMPUTE=false set. Otherwise they didn't take effect. Interestingly, the traces were updated when hitting plus or minus on the keyboard. This is maybe some hint for how things should be changed/fixed.

But in any case, we should probably do a comprehensive check of visual output with precompute mode on and off, for at least the following behaviors:

1. Enabling/disabling projectors
2. Clicking/unclicking bad channels that are in projectors when projection is on (as this changes the projector that is applied)
3. Changing channel scalings using plus/minus on the keyboard
4. Changing channel scalings using the new channel-specific UI

@nmarkowitz for now you don't need to worry about any of this stuff -- I just need to remember to test with MNE_BROWSER_PRECOMPUTE=false (which I'll do by setting my system config value now).

[larsoner]

Whitened channels and maybe stim channels are scaled differently in _process_data in MNE, too, so we need to make sure we treat them properly here.