Remove unused var "total"
Declare vars as locally as possible

issue #2678

On some platforms sin and cos are calculated in single precision resulting in
diversity of results.

Old implementation did

    lut[i] = 255 * (count(Y <= i)) / (width * height)

which actually shifts uniform histograms.
From now histogram is equalized as

    C = count(Y == min(Y))
    lut[i] = 255 * (count(Y <= i) - C) / (width * height - C)

disabled several tests on Mac when no FFMPEG is used; disabled automatic table initialization in imgwarp in the case of MinGW

Also removed assigned only variable.

[edit: cleaned whitespace]

This reverts commit 4aaaef59.

Conflicts:
    modules/imgproc/include/opencv2/imgproc/types_c.h
    modules/legacy/include/opencv2/legacy/legacy.hpp

This partly reverts commit 6ca61827.

added recommended number of stripes to parallel_for_, modified some of the functions to use larger stripes (for better performance)

expanded cv::threshold parallelization to other threading frameworks; fixed potential bug with unprocessed bottom of the image; fixed build problem in stitching