Mser sample improvments (#12032)

* Fixed bug in detect_mser sample
Wrong number of colors used to generate the synthetic images

* Formatting improvements

* Using safer casts

* Improved readability of legend generation

* Various readability fixes in detect_mser sample

- use per-module includes instead of "opencv.hpp"
- unused function warnings
- undefined macro warning

Original commit is a5f19f7dd6ae5bc93f73e0417a9e0bfd34c01672

* Solve issue 7676

* I7676 bis

All of these: (performance) Prefer prefix ++/-- operators for non-primitive types.
[modules/calib3d/src/fundam.cpp:1049] -> [modules/calib3d/src/fundam.cpp:1049]: (style) Same expression on both sides of '&&'.

An example how to use features2d for MSER. Data results are visualized in 3D using openglwith mouse or keyboard