Add an option to change framerate mid run

[Matte27]

Low priority but would be nice for meatboy all glitches

[clementgallet]

Most of the work done in 337fc9c

Still a few things to take care of:

• Encoding is not supported
• If there are computations of the movie length, they are now incorrect (check movie metadata)
• I'm resetting the sub-nanosecond accumulator (used to compute the length of a frame) when framerate has changed, is that a problem? This accumulator is used when the length of a frame cannot be represented as a integer number of nanoseconds (smallest unit of time used in Linux), but the meaning of its value depends on the value of the framerate.
• Displayed values when loading a savestate are incorrect (they look correct after advancing one frame)
• Check if audio mixing is holding up

[clementgallet]

• Fix the interaction with restart/auto_restart