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.

video/out/gl_common.c

@@ -141,6 +141,11 @@ static bool is_software_gl(GL *gl)
            strcmp(renderer, "Mesa X11") == 0;
 }
 
+static void dummy_glBindFramebuffer(GLenum target, GLuint framebuffer)
+{
+    assert(framebuffer == 0);
+}
+
 #define FN_OFFS(name) offsetof(GL, name)
 
 #define DEF_FN(name)            {FN_OFFS(name), {"gl" # name}}
@@ -618,6 +623,10 @@ void mpgl_load_functions(GL *gl, void *(*getProcAddress)(const GLubyte *),
 
     mp_verbose(log, "Detected OpenGL features:");
     list_features(gl->mpgl_caps, log, MSGL_V, false);
+
+    // Provided for simpler handling if no framebuffer support is available.
+    if (!gl->BindFramebuffer)
+        gl->BindFramebuffer = &dummy_glBindFramebuffer;
 }
 
 /**