Making /dev/dri support optional to deal with special hardware cases

[cricalix]

In #1204, the OP indicated they've got a motherboard the presents /dev/dri/renderD128, but it's not functional in the container and breaks the setup of the container before it even gets to the point of installing the application.

Looking at what I've seen of the installation flow, it should be possible to

1. add a variable for enabling transcode support
2. add a whiptail section in advanced setup (if the app name matches the current transcode-device-desired list used elsewhere?) to allow changing that setting
3. respect that setting in build.func to totally disable creating any pass through or mount of the devices
4. remove the checks for $APP being one of a hard-coded list in lieu of the $var_transcode_support check (in build.func ~830)

This would be a relatively small PR.

However, I think there's a way that might be better. My criteria for better is "remove the hard-coded check by app name completely, instead of moving it to a whiptail dialog gating check".

1. Add new variable, default false, var_can_use_transcode
2. Add a new variable, default false, var_enable_transcode
3. Update all app scripts listed in build.func around line 840 to default both variables to true
4. Add a whiptail dialog in advanced setup that gates on var_can_use_transcode being true/1, allows changing of var_enable_transcode
5. Update build.func's code to not have the long list of OR checks for app name, instead gate on var_enable_transcode being true/1
6. Keep the /dev/dri checks, but if enable was true, add a warning if dev/dri* is missing?

This would be a bit of a larger PR.

Folks who steward this project - if either path seems acceptable, I can produce some PRs in the next week or so; work starts again tomorrow, so my time (and enthusiasm) for working on this stuff after my daytime hours will reduce (as I write code all day long).

[cricalix]

For clarity on the second option, I'm suggesting two variables so that the branch to disable transcode is only applied when the app script has what could be called a feature flag set. This means the advanced settings would only show it for apps that can even use transcoding.