Stream sources are traited as GMats in the G-API's internal type system

[dmatveev]

As mentioned in opencv/opencv#24178 (namely, gtype_traits.hpp#L147), g_type_of_t<IStreamSource::Ptr> resolves to GMat, however the source itself may produce data of any G-API kind (GMat / GFrame / GScalar / GAarray / etc).

As shown in the same PR, creating a Queue Stream source producing a Scalar still works and works correctly, but internally we have such an inconsistency.

g_type_of_t<> is used to identify the right host data type wrapper helper:

1. It is used in cv::gin() which accepts data of any kind;
2. It is used to pack this data to a GRunArg form (which is a variant over a set of predefined types);
3. Some host types like Mat and Scalar may be wrapped as-is, but some (normally - a customizable template ones) require special handling to strip type and wrap value into a container like VectorRef or OpaqueRef (and store some type information in that container).

When an instance of IStreamSource::Ptr is passed to cv::gin(), treating it as cv::GMat in this particular case means wrapping it using a default helper, no actual GMat-specific handling is done. Anyway, it may be confusing to resolve g_type_of_t<> to GMat in this case and this disambiguation needs to be resolved.