Require standard library implementers to provide support for image surfaces on all platforms

[mikebmcl]

Providing output surface support will be encouraged but not required. This is a good compromise since there are many use cases where image surfaces alone will suffice (e.g. generating graphics for reports and writing image manipulation software) and its possible to write platform independent support for image surfaces whereas output surfaces inherently depend on platform-specific details. This is analogous to how the C standard distinguishes between file I/O and console I/O in the stdio.h function specifications. This issue supersedes #175 and as such this closes #175 .