Correctly handle non-360° pictures in the interface

There are two main problems with non-360° pictures right now:

• the picture is repeated horizontally, even though the left-most and right-most edges do not match

• when specifying a given azimuth and elevation (for instance using the control box), if the azimuth is outside the image, then the resulting position is completely wrong. In this case, the position should be cropped to the nearest image boundary (exactly like it is already done vertically, for elevation).

The second issue is fixed by 26a271f1.

For the first issue, we can distinguish between two cases:

• the picture is "almost" 360°. In this case, we could add some space without tiles that represents the missing bit of photography. This way, the panorama would correctly loop in the interface.

• the picture is far from being 360°. In this case, it wouldn't be very user-friendly to add a large portion of untiled space to fill the gap to 360°. But we could still add a bit of untiled space on each side, so that reference points or other panoramas are shown on the interface even though they are just a bit outside of the picture.