PFNGLCOPYTEXSUBIMAGE3DPROC

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 PFNGLCOPYTEXSUBIMAGE3DPROC = void function(GLenum target, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLint x, GLint y, GLsizei width, GLsizei height)

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