const-corrections

~~Both `enableTextureTarget()` and `disableTextureTarget()` might raise some questions about their `const`-ness, but those member functions (which conditionally access an outside global state) are kind of awkwardly situated to begin with.

This is API changing and will likely break any addons that extend the various interfaces and helper classes (such as `ofVideoPlayer`).~~

... in the meantime, this evolved into a little bigger (but still important project).  I would propose that we not try to fix every little const problem in one PR, but this one can focus on video and images/pixels/textures, which are tightly coupled.