Support Custom Graphics Extensions

[jojo2357]

It was mentioned in #3145 and I want to do it, but it seems hard...RequiredFileArgument's extensions are very static. Any suggestions @PHPirates ?

[PHPirates]

I'm not sure what issue you are referring to?

[jojo2357]

Support DeclareGraphicsExtensions, sorry forgot to mention it explicitly.

[PHPirates]

The most difficult thing about \DeclareGraphicsExtensions I think is that you can use it anywhere in a document taking only effect from then on. But if we ignore that, it might be doable. If we need to resolve a file then we already are using indexes so I hope the performance will not be much worse.

How I would implement it I think, is in InputFileReference when resolving an \includegraphics, first check the LatexCommandsIndex for any \DeclareGraphicsExtension and if found use that order of preference given to findFile.

In case performance turns out to be bad, we could first check if there are multiple files with the same name, which is usually not the case, and only if there are do the expensive check.