KTXwriterScParams support

[aqnuep]

Fixes #758.

Depends on the CTS PR KhronosGroup/KTX-Software-CTS#5.

@MarkCallow FYI (please make sure you merge the CTS PR first with rebasing policy).

[MarkCallow]

Please rebase to get the MinGW fix.

Looks good but the repetition of so many string values is unfortunate. Is there a way to reduce the repetition? If not, please define a const string object for each value and refer to that at each place where needed.

[aqnuep]

Please rebase to get the MinGW fix.

Looks good but the repetition of so many string values is unfortunate. Is there a way to reduce the repetition? If not, please define a const string object for each value and refer to that at each place where needed.

Yeah, I tried to minimize that, in fact eliminated one of them. We could use some consts, sure.

[MarkCallow]

Yeah, I tried to minimize that, in fact eliminated one of them. We could use some consts, sure.

Please do it.

Note that my e-mail service is currently broken. I can neither send nor receive messages from my regular addresses. Therefore my responses may be somewhat delayed.

[aqnuep]

I've rebased and added string constants for all the places where it made sense. This should be fine once CI passes.

I can neither send nor receive messages from my regular addresses.

Is there any channel through which we can contact you privately in the meantime?

[MarkCallow]

Is there any channel through which we can contact you privately in the meantime?

I've just sent you a message from it.

[aqnuep]

Removed "astc-mode" option, as requested. Instead, default to LDR mode for UNORM/SRGB.

[aqnuep]

Updated CTS git submodule hash. Ready for merge.