Universal Windows Platform support

[atteneder]

Had to make OpenGL upload functions optional to get libktx to build for UWP.

[MarkCallow]

Is this actually necessary? glloader.c doesn't attempt to do anything unless the application calls the function and all it needs for building are the header files included in other_include. If the platform doesn't support OpenGL then the app won't be able to create a GL context which is a prerequisite to calling the ktxTexture[12]_GLUpload. Of course it will save a little space if it's not there and will avoid confusion.

[atteneder]

Can't remember exactly, but when creating this fix initially (against beta 6 iirc) I think it was necessary. Let me check.

Since we're at it: are there plans to implement similar upload functionality for further graphic APIs (Metal, Direct3D)?

[atteneder]

Sorry, I should've tested more thoroughly before submitting the PR.

I tried to configure/build but got lots of errors liket this one. Looks like the default compiler configuration for this platform is more strict. You mentioned you're working on level 4 warning stuff, so I'll wait until that landed before I continue.

In case you want to try out UWP/ARM64, here's how I configured it:

cmake . -G "Visual Studio 16 2019" -A ARM64 -B build_uwp_arm64 -DCMAKE_SYSTEM_NAME:String=WindowsStore -DCMAKE_SYSTEM_VERSION:String="10.0" -DBASISU_SUPPORT_SSE=OFF

Thanks

[MarkCallow]

Since we're at it: are there plans to implement similar upload functionality for further graphic APIs (Metal, Direct3D)?

I have no plan but I'd happily accept contributions.

Sorry, I should've tested more thoroughly before submitting the PR.

It's building now without warnings. What did you do?

[MarkCallow]

I've landed PR #418 raising the warning level and fixing all the warnings. Please rebase your PRs.

[MarkCallow]

Please add how to build for Windows Store to BUILDING.md. Then I'll merge this.

[atteneder]

I added build instructions and fixed the cpu detection when cross compiling to UWP ARM/ARM64 (so that SSE is not mistakenly enabled)

[MarkCallow]

Please fix the conflicts. They're probably because I just landed the Android build PR.