PFNGLTEXTURESTORAGE3DEXTPROC

Undocumented in source but is binding to C. You might be able to learn more by searching the web for its name.
extern (C)
alias PFNGLTEXTURESTORAGE3DEXTPROC = void function(GLuint texture, GLenum target, GLsizei levels, GLenum internalformat, GLsizei width, GLsizei height, GLsizei depth)

Meta