Modify or eliminate the type aliases that simplify basic_ to ensure that standard library implementers aren't required to provide them

[mikebmcl]

Probably eliminate since otherwise it would become inconsistent, but this needs thought. The type aliases referred to are the ones that remove basic_ and provide the required template arguments such that they make the class templates look like they are classes.

[mikebmcl]

My thinking is that standard library implementers should only be required to provide the front end, i.e. the basic_* class templates and a back end that is allowed to fail in certain ways. They should be required to provide support for image surfaces and all operations on them. That can all be done in software such that there's no requirement for anything platform-specific. As such, all back end implementers should definitely be allowed to support only image surfaces. Output surfaces should be optional for back ends. Back ends that support output surfaces for all major platforms will almost certainly emerge (the reference implementation already supports all major platforms except Android and adding support for that is a high priority).

Any requirement that output surfaces be supported vastly increases the burden on implementers, even if its just a requirement to do so for at least one platform. A counter argument to not requiring such support is that if there's no requirement to provide output surfaces, users can't rely on being able to use the library for user-visible output. The reply to that is that this is already true for all non-standard libraries, that providing a standard 2D graphics API (even if it is only guaranteed for image surfaces) is a significant improvement over the status quo, and that the TS process will show whether or not the expectation that back ends that support output surfaces for all major platforms (and perhaps even for more niche platforms) is valid.

[mikebmcl]

Superseded by #177.