Implement multi-monitor rendering support

[kblaschke]

As requested multiple times by users over the past weeks, adding proper multi-monitor fullscreen support should be considered. Due to today's monitor sizes and different setups, simply stretching a single, borderless window over all displays won't be a great solution. Specifically, the maximum texture size may not be enough if a user arranges 3 4K monitors side by side for example, as many GPUs only support up to 8192×8192 pixels.

My proposal here would be the following:

• Let the user configure on which displays projectM should be rendered on, ideally remembering the display name or port if we can get this from SDL.
• Let the user decide to either stretch or clone the output to the selected displays. Cloning is nice for beamer/LED screen display with a local reference monitor.
• For stretching, add a render resolution selection. If automatic, check maximum texture size and limit the render size accordingly, then stretch as needed.
• One output window should be the main one, which will render the UI.

This will require creating several output windows sharing an OpenGL context, and rendering to an off-screen surface. Then glBlitFramebuffer() can be used to copy the required image parts to each output window, eventually scaling it up.

[kblaschke]

In addition to the above, if performance allows, projectMSDL could also run multiple projectM instances, showing each on a single monitor. This would allow showing different presets, but keeping the transitions synced.