Compute mouse and finger coordinates using renderer output size instead of window size

(Partly reverts 9460e64c) + (Issue #1340 (closed) remains fixed)

In the Lua API, the sol.video.get_window_size() function should always return the windowed size, even if in fullscreen, in order to stay symmetrical with sol.video.set_window_size(), i.e. set and get must remain consistent.

Mouse and finger coordinates are now computed using get_output_size() instead of get_window_size() to properly restore the behaviour of the latter.

The get_viewport() function is not used anywhere else and therefore is now also merged into the renamed output_to_quest_coordinates() for consistency.

@stdgregwar I tested this on RPI non-X, when you have time can you please test on your devices if #1340 (closed) effectively remains fixed aswell there? Thanks.