m_option: fix -vo opengl lscale validation

OPT_STRING_VALIDATE actually did nothing. This made -vo opengl crash or
misbehave when passing an invalid value for the lscale, cscale or 3dlut-
size (the only users for this option type).

The code added with this commit was either blatantly forgotten with the
commit introducing this option type, or somehow lost.
gl_video: explicitly clamp colormatrix output

This could lead to quite visible artifacts when using an appropriate ICC
and float FBOs. The float FBOs allow storing out of range values, and my
guess is that the rest of the precessing chain elevated these out of
range values, resulting in artifacts.
gl_video: reduce output with -v, skip useless header parts

Originally, the header wasn't supposed to contain random compatibility
stuff, but now all that is printed with -v. Add a hack to skip it and
to reduce the noise.