Allow overloads of the same function

It is now possible to define multiple overloads of the same function,
each with different parameters. This is extremely useful for
maintaining backwards compatibility in the face of the changes between
GL 4.3 and 4.4.
Added PrimitiveType <=> BeginMode overloads

Khronos renamed BeginMode to PrimitiveType starting with GL 4.4.
OpenTK now has overloads for both flavors.