Fix left shift on signed int channel setting sign bit and causing UB

[walcht]

I don't whether this was causing any issues, but:

• When the signed integer (type: int) is 192, the following undefined behavior occurs: 11000000 << 24 => is undefined behavior because setting sign bit of an integer via a left shift operation is UB. Simply cast channel to uint32_t before shifting.

• To replicate: build with -fsanitize="undefined" and navigate to KTX-Software/tests/cts/clitests then:
  ktx transcode --testrun input/ktx2/valid_R16G16_SFLOAT_CUBE_ARRAY.ktx2 / output/transcode/transcode_error_not_transcodable/output_R16G16_SFLOAT_CUBE_ARRAY.ktx2

You will get the following:

KTX-Software/external/dfdutils/createdfd.c:123:18: runtime error: left shift of 192 by 24 places cannot be represented in type 'int'

[MarkCallow]

The sampleNo and channel parameters should really be unsigned but I have not looked to see what the knock-on effects of that change would be. There is no reason for those values to ever be signed.

[walcht]

Agree. That cast from int->uint32_t is safe (channel at most sets the 32-24=8th bit). I didn't want to introduce any other changes (e.g., setting channel as uint32_t in parameter list).