allow bayer-denoise ISP block to be disabled

[karpawich]

The PiCamera can be used as a low-cost and widely accessible entropy source for RNG (random number generation) by calibrating camera settings to maximize noise (see the LavaRnd website for more details). Previously, developers could only disable the secondary YUV420 de-noise algorithms.

Per #565 'bayer_denoise': 1 << 10 can be added to the ISP_BLOCKS object so that it can be disabled via the PiCamera isp_blocks parameter.

To address the concerns from @6by9 about disrupting the image processing pipeline, disabling the Bayer de-noise ISP block would not stall it because image format is preserved, similar to the YUV420 de-noise algorithm.

[6by9]

To address the concerns from @6by9 about disrupting the image processing pipeline, disabling the Bayer de-noise ISP block would not stall it because image format is preserved, similar to the YUV420 de-noise algorithm.

Preserving the image format has nothing to do with my concerns.
The ISP works on tiles, and several of the processing stages require context pixels around those that it is actually processing. You're using configurations that have not been exercised before, therefore there is the potential for bugs to be lurking in those tile calculations.
If you're saying that it all works then that is great, but if there are issues then they won't be high on my priority list to fix.

Suffice to say that the patch looks reasonable to me, however this isn't my repository.