vo_opengl: enable dumb-mode automatically if possible

I decided that I actually can't stand how vo_opengl unnecessarily puts
the video through 3 shader stages (instead of 1). Thus, what was meant
to be a fallback for weak OpenGL implementations, the dumb-mode, now
becomes default if the user settings allow it.

The code required to check for the settings isn't so wild, so I guess
it's manageable. I still hope that one day, our rendering logic can
generate ideal shader stages for this case too.

Note that in theory, dumb-mode could be reenabled at runtime due to a
color management 3D LUT being set, so a separate dumb_mode field is
required. The dumb-mode option can't just be overwritten.

DOCS/man/vo.rst

@@ -311,7 +311,10 @@ Available video output drivers are:
         of bad or old hardware.
 
         This mode is forced automatically if needed, and this option is mostly
-        useful for debugging.
+        useful for debugging. It's also enabled automatically if nothing uses
+        features which require FBOs.
+
+        This option might be silently removed in the future.
 
     ``scale=<filter>``