add imageIndex/levelIndex to getHasAlpha

[georgios-uber]

No description provided.

[richgel999]

Hi - The change itself looks good, although it's important to note that whether or not a .basis file has alpha is a property of the entire file, not a specific level/index.
If you look at basis_transcoder::get_image_level_info(), you'll see that the alpha_flag comes from the header itself:

image_info.m_alpha_flag = (pHeader->m_flags & cBASISHeaderFlagHasAlphaSlices) != 0;

So I could accept this change, but it wouldn't really add anything. Thoughts?

[richgel999]

I'm leaving this PR up for investigation.