vo_opengl_old: fix OSD regression

Commit 0e8fbdbd removed the rg_texture requirement from vo_opengl;
commit 541f6731 changed to a more convenient method. Both commits broke
vo_opengl_old in some ways. vo_opengl_old always requires GL_ALPHA for
single-channel texture, because it draws the OSD without shaders and by
using certain blend modes.

So we need to explicitly distinguish between vo_opengl and vo_opengl_old
in the OSD renderer, and force fixed texture formats for vo_opengl_old.
The other logic is specific to the internals of vo_opengl. (Although it
might be possible to get the same result by playing with the old GL
fixed-function functions in vo_opengl_old. But seems like a waste of
time.)

Fixes #1370.

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