Remove normalization in cv::stereo::QuasiDenseStereo getDisparity().

[dimitrisPs]

This addresses user comments made in #1941 and #2819. The getDisparity function, of QuasiDenseStereo, originally returned a disparity image normalized to span the full range between 0 and 255, which was improper and understandably confused users.

I've made the following changes:

• getDisparity now returns proper CV_32F disparity maps with nan values for unknown matches.
• included an accuracy test.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• 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 other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to 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

[alalek]

Thank you for contribution!

[alalek]

Please use short license header: https://github.com/opencv/opencv/wiki/Coding_Style_Guide#file-structure

// This file is part of OpenCV project.
// It is subject to the license terms in the LICENSE file found in the top-level directory
// of this distribution and at http://opencv.org/license.html.

[alalek]

cvtest::BaseTest

BTW, It makes sence to avoid using of legacy OpenCV testing infrastructure.
Prefer to use GoogleTest directly.

Start from this and then replace legacy calls (like printf => EXPECT_*() / ASSERT_*()):

-class
 ...

-void CV_QdsMatchingTest::run(int)
+TEST(qds_matching_simple_test, accuracy)
 ...

-TEST(qds_matching_simple_test, accuracy) { CV_QdsMatchingTest test; test.safe_run(); }

[dimitrisPs]

Sorry for that, I used the test files from the other stereo modules as templates, when I was writing qds' test, assuming that those follow the guidelines. I will make changes later this week, thanks for your help.