vo_opengl: handle grayscale input better, add YA16 support

Simply clamp off the U/V components in the colormatrix, instead of doing
something special in the shader.

Also, since YA8/YA16 gave a plane_bits value of 16/32, and a colormatrix
calculation overflowed with 32, add a component_bits field to the image
format descriptor, which for YA8/YA16 returns 8/16 (the wrong value had
no bad consequences otherwise).

video/csputils.h

@@ -116,6 +116,8 @@ struct mp_csp_params {
     float rgamma;
     float ggamma;
     float bgamma;
+    // discard U/V components
+    bool gray;
     // texture_bits/input_bits is for rescaling fixed point input to range [0,1]
     int texture_bits;
     int input_bits;