Many extension constants are written to external.go

[bbredesen]

The intent of exten.go is to group the constant name and spec version strings for all extensions. Some values are written to exten.go, but the majority are written into external.go. This bug does not appear to be present in platform specific extensions (i.e. exten_win32.go has all of the win32 extension values), but that could be a sampling error d/t the relatively small number of platform extensions.

Some extensions are even split between the two files, i.e. spec version is in exten.go and extension name is in external.go.

The quantity and specific set of extensions that are written in one or another file is different on each run of vk-gen, even with the same input file.