Enable BCn texture support on iOS where available

[TellowKrinkle]

So all those M1 iPads can enjoy their high res texture packs

Note: This means Xcode 14.3 will be required for anyone compiling for iOS. Should I add anything to cmake for that?

[OatmealDome]

re: Xcode requirement

I think it would be a good idea even for upstream, since we do technically have one but we don't explicitly state what it is anywhere (I don't even know what it is either). We give an error when Visual Studio is outdated, so why not do the same for Xcode?

[TellowKrinkle]

Okay, I stuck in the minimum Xcode version (which appears to be 13.0 for targeting macOS)

I also updated the minimum GCC to 10, as we now use consteval which requires 10. Haven't actually tested 10, so the minimum could be higher though.

[OatmealDome]

Hmm, I didn't expect compiler version checks for some reason, but it makes perfect sense. We have #10969 for this already, which is more complete (covers more compilers), but I have no idea about its status.

Could you drop the Xcode and GCC commits from this PR? I'll merge it right after, and then we can figure out what's going on with the compiler version stuff. Sorry about the hassle.