core: inconsistency of addition for multi-channel Mat and Scalar

[alalek]

OpenCV 3.4.9 / 4.2.0:

arg1 - uint8 matrix with 3 channels:

• Mat m__1_2_3(Size(2, 2), CV_8UC3, Scalar(1, 2, 3));

arg2:

• 5
• Scalar(5) (identical to Scalar(5, 0, 0, 0))
• Scalar(5, 1e-6, 0)
• Scalar::all(5) (identical to Scalar(5, 5, 5, 5))

Result variants:

1. [6, 2, 3] - modified 1st channel only
2. [6, 7, 8] - all channels are modified

#	C++ code	arg1 5	arg2 Scalar(5)	arg3 Scalar (5, 1e-6, 0)	arg4 Scalar::all(5)
1	cv::add(arg1, arg2, res)	2	1	1	2
2	MatExpr res = arg1 + arg2	1 #14738	1	1	2
3	MatExpr res = 1.001 * arg1 + arg2	2 #14738	2 #16538	1	2

Analyse:

• results with Scalar(5, 1e-6, 0)(arg3) and Scalar::all(5)(arg4) are consistent and adequate
• some other cases should be be deprecated / changed / removed?
• case with numeric scalar (5) may have designed to use with complex numbers (emulation through matrices with 2 channels) where 5 is interpreted as a real number.
• current implicit conversion of 5 into Scalar (no explicit bug/feature) converts this into Scalar(5, 0, 0, 0). Extra overloads may help to avoid implicit conversions (but may break user code).

Related unclear stuff:

• multi-channel Mat::eye() without element initializer.
• missing explicit for Vec and Scalar constructors: cv::Matx33f tmp = true compiles #11956
• almost zero tests for MatExpr stuff

[saskatchewancatch]

Looking at the Jiras, it seems many people expect OpenCV matrix's to behave just like in Matlab.
Perhaps behaviour should strive to be as close as possible to the way Matlab treats Matrix's?