Some non-trivial tricks had to be done in order to expose the
GL::defaultFramebuffer variable without causing leaks or double frees.
Also, the AbstractFramebuffer has a protected destructor so this needs
another special handling.
Only the double ones, exposed as floats, because the extra ALU required
by doubles is negligible to function call overhead. It'll be different
for non-scalar types, but here I use this.
Vladimír Vondruš