Faster distance calculation between multigeometries

[patrickbr]

This so fare includes several successful experiments I did over the last week, with the overall goal of speeding up the distance calculation between very large multi-geometries (MULTI* and COLLECTION).

1.) Store the current upper distance bound across multi geometry parts, which causes many candidates being already discarded via a simple bounding box distance calculation

2.) Improve the initial upper bound by storing, for each multigeometry, the rightmost point during parsing

3.) Make the padding for the meter distance distortion on the web mercator projected plane more tight by only considering the local distortion (e.g. if we know that both geometries are on the same "band" around the globe, it is necessary to consider the global distortion factor initially for computing the euclidean distance upper bound, but for the further sweep padding it is then enough to only consider the relative distortion between the geometries (which is nearly 1 if latitudes are close)

4.) Update libspatialjoin to a new experimental version which computes tighter initial upper distance bounds on start (by doing a simple probing of 4 points) and which also updates the sweeping padding dynamically during the sweep.

5.) On the side, fix a very subtle bug in which the distortion for the candidate sweep padding was computed based on the original geometry, but it must be computed based on the padded geometry. This caused some lost candidates on very large distances.