opencv: cvequalizehist: add color support

Submitted by ji0..@..ng.com

Link to original bug (#748858)

Description

Created attachment 302826
patch for extending an input format

The original "cvequalizehist" element only accepts a grayscale image as an input.
However, the most images users want to handle are colored images.
I extended its input format from grayscale to colored.
Please find the attached patch.

This patch was modified from d4703fa8,
and you can check it using the following command.

gst-launch-1.0 videotestsrc pattern=13 ! tee name=t t. ! queue ! videoconvert ! xvimagesink t. ! queue ! cvequalizehist ! videoconvert ! xvimagesink

Thank you.

Patch 302826, "patch for extending an input format":
equalize_hist_colored_image.diff

Version: 1.x