This structure represents an offscreen context
draw buffer. It has a framebuffer, with at least
color renderbuffer (alpha or not). It may also have
packed or independent depth or stencil buffers,
depending on context requirements.
This is a cross-platform struct, that every GLContext implementation
should have under the field capabilities, as a public field
This should allow us to know the capabilities of a given
GLContext without repeating the same code over and over