support saving 32FC1 as tiff image, fix issue 7762

[jasjuang]

fix issue #7762

[alalek]

img.convertTo(img8U, CV_8UC1);

Implicit unexpected data loss in encoders must be avoided.

IMHO, isFormatSupported should be extended to support channels information too (possible problems related to ABI should be workarounded).

[vpisarev]

it may be also confusing, because some people assume that floating-point images have value range [0, 1], i.e. 0 is black and 1 is white, not [0, 255]. So, this patch cannot be accepted in the current form

[jasjuang]

@vpisarev OpenCV is able to save CV_32FC1 mat into pngs and jpgs as if it's CV_8UC1, and this is verified by reading it back with imread. I am not sure how OpenCV handles it, but couldn't we apply the same process to tiffs?

[vpisarev]

We decided not to put such implicit conversion into OpenCV. Please, call Mat::convertTo() prior to imwrite().