Cleanup the video plugins

[ColinPitrat]

With the migration to SDL2 (#185) some video plugins (The two HW ones) become redundant.
Moreover, the bicubic plugin is slow even on the fastest hardware and could be removed (#160)

The main difficulty is that removing them without care could break the configuration (the index of the video plugin in the array is the number present in the configuration).

We would need some kind of indirection mapping the number in the configuration to a given entry in the array of plugins. Typically, 0 and 2 would both point to half plugin, 1 and 3 would both point to double plugin, etc ... The one corresponding to bicubic could point to bilinear instead.

[ColinPitrat]

Done with a "hidden" boolean in vid_plugin definition.