vo_opengl: don't crash if framebuffers are not available

In theory, vo_opengl supports operation without framebuffers. But this
has been broken for a while now (commit cc00b3ff is a contender). It
crashed because it unconditionally called gl->BindFramebuffer() (which
is NULL if framebuffers are missing).

Since this function is actually only called to set the default
framebuffer, the simplest way to deal with this is to provide a dummy
function, insteas of uglifying the code with additional if branches.
vo_opengl: make shader text output slightly nicer

It was missing an indentation in some cases.