check whether API supports requested bit-depth

[farindk]

This adds the check to kvz_api_get(bit_depth) whether the requested bit depth is supported.
See #399

[Jovasa]

The 10bit kvazaar does support bitdepths <= 10, so LTE check is more appropriate than inequality check

[farindk]

I just tried 8bit encoding when setting KVZ_BIT_DEPTH=10.

The problem with that is that when the bit-depth is set to 10, it assumes that the input image is 2 bytes per pixel even if we use kvz_api_get(8). That's a bit unfortunate. A client that wants to encode 8 bit would then have to query if 10 bits is supported and depending on that it has to change the input image.

[Jovasa]

I fully agree with your concern, the main issue is that this change would break existing behaviour.

@fador opinions?