Consider ways to provide users with a runtime check to see if graphics is supported at all and if so whether output surfaces are supported or only image surfaces

[mikebmcl]

Some platforms might only be able to provide support for image surfaces and some programs might only require support for image surfaces, e.g. programs that generate reports that include graphics. The mechanism itself can be as simple as having an attempt to create an output surface fail, but the specification of the back ends (i.e. the template requirements) needs to make this clear and there should be some way for users to differentiate between an attempt to create an output surface failing because they asked for something they system couldn't do because it doesn't support any output surfaces versus failure for some other reason.