This PR adds the Floor slicing feature for the bsp2svg tool

disabled by default
it includes automatic floor detection, the detection parameters can be adjusted with arguments
before / after screenshot - the saint.bsp map from Honey, with the top floor hidden https://imgsli.com/MjIwNDIx
if there is more than one floor detected / configured, the export will also include a hidden layer that has all floors (identical to the old export)

Examples

bsp2svg.exe e4m1.bsp -s enables slicing with everything default
bsp2svg.exe e1m1.bsp -s -t 1 0.25 32 slices with different floor detection parameters, by the default axis (z)
bsp2svg.exe alk_tellus.bsp -s -t 1 0.2 96 -i flat01 as above, but different parameters and an extra ignored texture
bsp2svg.exe alk_tellus.bsp -s -p y -l z -t 1 0.2 96 -i flat01 as above, and additionally projected by the y axis, sliced by z
bsp2svg.exe ad_start.bsp -s -p x -l z -t 1 0.5 32
bsp2svg.exe elders7.bsp -s -p y -l z -t 1 0.15 32
bsp2svg.exe saint.bsp -s -t 1 0.2 96
bsp2svg.exe qbj2_start.bsp -s -p y -t 1 0.2 64
bsp2svg.exe ad_tears.bsp -p x -t 1 0.2 96 -s