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

the static variables will cause race-condition when operating in
multithread scenarios.
Signed-off-by: NTeng Yiliang <ylteng@outlook.com>
Signed-off-by: NTeng Yiliang <yiliang.teng@weimob.com>

  * replace binary literal prefix to hexadecimal literal prefix

Example (reuse 'Transform' test case):
PERF_TEST(Transform, getPerspectiveTransform_1000) { ... }
PERF_TEST(Transform, getPerspectiveTransform_QR_1000) { ... }