vo_opengl: refactor pass_read_video and texture binding

This is a pretty major rewrite of the internal texture binding
mechanic, which makes it more flexible.

In general, the difference between the old and current approaches is
that now, all texture description is held in a struct img_tex and only
explicitly bound with pass_bind. (Once bound, a texture unit is assumed
to be set in stone and no longer tied to the img_tex)

This approach makes the code inside pass_read_video significantly more
flexible and cuts down on the number of weird special cases and
spaghetti logic.

It also has some improvements, e.g. cutting down greatly on the number
of unnecessary conversion passes inside pass_read_video (which was
previously mostly done to cope with the fact that the alternative would
have resulted in a combinatorial explosion of code complexity).

Some other notable changes (and potential improvements):

- texture expansion is now *always* handled in pass_read_video, and the
  colormatrix never does this anymore. (Which means the code could
  probably be removed from the colormatrix generation logic, modulo some
  other VOs)

- struct fbo_tex now stores both its "physical" and "logical"
  (configured) size, which cuts down on the amount of width/height
  baggage on some function calls

- vo_opengl can now technically support textures with different bit
  depths (e.g. 10 bit luma, 8 bit chroma) - but the APIs it queries
  inside img_format.c doesn't export this (nor does ffmpeg support it,
  really) so the status quo of using the same tex_mul for all planes is
  kept.

- dumb_mode is now only needed because of the indirect_fbo being in the
  main rendering pipeline. If we reintroduce p->use_indirect and thread
  a transform through the entire program this could be skipped where
  unnecessary, allowing for the removal of dumb_mode. But I'm not sure
  how to do this in a clean way. (Which is part of why it got introduced
  to begin with)

- It would be trivial to resurrect source-shader now (it would just be
  one extra 'if' inside pass_read_video).

video/img_format.h

@@ -93,6 +93,7 @@ struct mp_imgfmt_desc {
     int8_t component_bits;       // number of bits per component (0 if uneven)
     int8_t component_full_bits;  // number of bits per component including
                                  // internal padding (0 if uneven)
+    int8_t components[MP_MAX_PLANES]; // number of components for each plane
     // chroma shifts per plane (provided for convenience with planar formats)
     int8_t xs[MP_MAX_PLANES];
     int8_t ys[MP_MAX_PLANES];