Add the Ability to Specify Egui Texture Filters

[zicklag]

Very quick draft, but it works for the egui_glow backend.

Not sure that it's a good API or not, and it needs more thought, but pushing this up because I needed it in my project, and other people might want to look at it.

[emilk]

I think this looks good overall!

I am keen to also add mipmapping as a filtering option, but let's save that for a later PR to keep down the complexity.

[zicklag]

Awesome!

I addressed most of your points and pushed an update, with just a couple of questions.

Edit: I've still got some build problems to fix, I'll get to that as soon as I can.

[zicklag]

Make another push, hopefully that fixes CI, sorry for the re-runs.

[emilk]

thanks!