For true offscreen rendering support, enable users to specify output size (resolution) independently from the frame buffer settings.

[NihanthCW]

Related to #1126

Although VAPOR supports offscreen rendering, the tight coupling between the visualizer resolution and custom output size set through Frame buffer settings, makes it awkward to use the application while attempting to export hi-res images.

Suggestion: Ideally, users should be able to specify output size (resolution) independently from the frame buffer settings. That way exporting a 4k image doesn't require the users to interact with a window that's displayed at 4k. Could use Blender's interface for inspiration.