cudacodec::VideoReader add colour format selection functionality

[cudawarped]

As mentioned in #3195 the frames returned from cudacodec::VideoReader::nextFrame/retrieve are always BGRA with a zero alpha channel. Regardless of the reason for this (likely memory optimization for previous generation gpu's and/or current OpenCV CUDA functions), GPU memory is at a premium and it seems like a reasonable idea to allow this to be changed to BGR and Gray if desired by the user.

It may impact performance of some CUDA routines if a BGR frame is used instead of BGRA but that would be up to the user to experiment with.

The default is left as BGRA for compatibility and the test has been updated.

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• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

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[alalek]

Thank you 👍